Forgery

Producing, replicating or altering a document and similar with a painting or sculpture are very different things. Each of the two "fraud" branches have their own methods, problems, and set of mechanics to govern the creation of these items and how difficult they are to detect to the observer. 
In each case, the use of a "kit" or a workstation, will be required to carry out the activity. 
== Forgery ==
In 5e there is the "Forgery Kit". This small box contains a variety of papers and parchments, pens and inks, seals and Sealing wax, gold and silver leaf, and other supplies necessary to create basic, but convincing, forgeries of physical documents. Proficiency with this kit lets you add your proficiency bonus to any Ability Checks you make to create a physical forgery of a document. 
Before any forgery can take place, a number of things need to be addressed. 
* An original sample of a person's handwriting, or signature, or a copy of a document being copied/altered will be required. - Getting a rogue's hands on these are adventures in and of themselves. Sometimes these can be purchased from other rogues or guilds. However, this can be dangerous as other rogues may have poor forgeries themselves, or have deliberately given a sample with errors so the rogue gets caught or killed. 
* Time, adequate lighting, and a writing surface are necessary. A rogue cannot write a "night pass" while running from the Watch, scribbled on some barreltop in a dark alley. No. Forgery is always a "long" game. Documents must be prepped first. 
* The lie that sells the forgery is often as important as the document itself. With access to common paperwork, a good Disguise skill, and a fast tongue, a Rogue can move at will through the environment if they are cautious and clever. 
There are concerns that need to be addressed about the actual documents as well. 
* How complex is this particular item to forge? A document with intricate printing, custom paper, metal woven into the paper, specialty dyes or other devices to prevent forgery is far more complex than a note written in somebody else's handwriting. It takes time, and money to duplicate expensive documents. You may have to find a source for the correct papers/inks if a government or bank is issuing custom documents or paper currencies.
* How much scrutiny will it be able to withstand? A forged gate pass in torchlight during a rainstorm isn't going to be eyeballed as hard as a deed certificate for the old mansion on the hill in some lawyer's well-lit chambers. 
* Are there other records to compare it to? - Documents are often created in duplicate or triplicate, and secured with various administrative bodies to keep a record of transactions. If the rogue has forged a Letter of Marque from the King, you can bet that the Royal Scribe will be consulted to ensure that there is an "official" copy on hand. The more important the document, the more likely there are many copies, sometimes even numbered or carrying marks to indicate which copy it is. Other problems are possible arcane bonds tying documents to locations or people, as well as any other possible law and judicial systems, such as Zones of Truth, and investigatory systems as people investigate the perjury. You might be able to scare a farmer off their land with a forged royal charter, but that doesn't stop them from going to a law enforcer. 
== Counterfeiting ==
There is no counterfeiting kit in the core, so we are inventing one. It is a large crate that contains tools and machinery, vials and stands, various fake arcane foci, and assorted bits that cannot be categorized, but are necessary for the creation of facsimile objects. Proficiency with this kit lets you add your proficiency bonus to any Ability checks you make to create a physical counterfeit of an object. 
Counterfeiting is a lot more involved than forgery. Anything can be copied, from paintings, to magic items, to machinery, to jewelry, to furniture. Literally anything, which makes this section a bit difficult to parse into something usable. 
Much like forgery, however, having the item on hand to copy is going to ensure that the counterfeiter has the best possible chance of success. In the case of coin currency, this is going to be relatively simple to obtain. A painting in a museum, not so much. The process of obtaining some way of getting a good look at the object to be counterfeited is an adventure in and of itself. 
Nearly all of the things mentioned in the forgery section, above, apply here. Adequate space, time, light and materials are needed to complete the task, and the scrutiny involved once the counterfeit is in place are important. Keep these in mind. 
== Methods ==
=== Forgery ===
Forgery requires writing materials appropriate to the document being forged, enough light or sufficient visual acuity to see the details of what you’re writing, wax for seals (if appropriate), and some time. 
* To forge a document on which the handwriting is not specific to a person (military orders, a government decree, a business ledger, or the like), you need only to have read through a similar document in detail, and you gain a +4 bonus on your check. 
* To forge a signature, you need an autograph of that person to copy, and you gain a +4 bonus on the check. To forge a longer document written in the hand of some particular person, an appropriate sample size of that person’s handwriting is needed.
* If you have the Observant, Diplomat, Silver-Tongued, or Investigator feat, you get a +2 bonus on Forgery checks.
The Forgery check is made secretly, so that you’re not sure how good your forgery is. Your Forgery check is opposed by the Investigation check of the person who examines the document to check its authenticity. The examiner gains modifiers on his or her check depending on certain factors. (see "Detection", below)
The Forgery check is Dex-based.
=== Try Again? ===
Usually, no. A retry is never possible after a particular reader detects a particular forgery. But the document created by the forger might still fool someone else. The result of a Forgery check for a particular document must be used for every instance of a different reader examining the document. 
=== Limitations ===
Forgery is language-dependent; thus, to forge documents and detect forgeries, you must be able to read and write the language in question.
=== DC ===
* A note on plain paper that's written in somebody else's handwriting might be a DC 8-14.
* A simple document/accounting book/other document that is common and written on, and with, readily available materials might be a DC 12-16.
* A writ/visa/official government issued document/bank certified check might be a DC 18-25.
*A deed to a property or lands can in some cases be written on special paper or with special inks, inlays or seals and might be a DC 18-30+.
=== One Last Word on Forgery ===
Official documents can provide an excellent means for a rogue to prove themselves, reinforce a point, argument, or attempt to persuade someone. With properly forged documents - created with a normal use of the Forgery skill and opposed by the viewer's Investigation check - the rogue can gain special bonuses to certain skills due to synergy. 
At the DM's discretion, the rogue can forge documents that grant a +2 circumstance bonus on a specific Deception, Intimidate, or Persuasion check. These false credentials become, in effect, the perfect tool for the job and provide a bonus much like the bonuses provided to other skills by masterwork tools. Unlike other bonuses, these apply only when the rogue presents the documents and the creature they are interacting with does not detect the forgery.
== Counterfeiting ==
Much like forgery, a check is made to determine the quality of the reproduction, but this is never tested until the object is observed by someone who has interest in the item. 
=== Action ===
Counterfeiting is not a short process. Even for coinage, you will need time to create something that doesn't look like a child did it. 
Rough time estimates: 
* A single coin may take several hours.
* A small painting may take a week.
* A piece of jewelry may take up to a few weeks.
* A large painting may take up to a month or more.
* A small sculpture may take a day, up to weeks months depending on materials.
* A large sculpture may take a week, up to several months depending on materials.
*A magic item can vary greatly between 1 month and several years.
The Counterfeiting check is Dex-based. 
=== Restriction ===
If there are special materials needed to complete the counterfeit, the Kit does not include them, and they must be obtained. This would include unique things like a certain kind of metal, wood, stone, wax, inlay, other decorative item(s), or anything that makes the object stand out. 
=== Special ===
If you have the Historian, Perceptive, or Quick-Fingered feat, you get a +2 bonus on Counterfeit checks.
If you have an example of the item in question you get a +4 bonus on the Counterfeit check.
=== DCs ===
* Modern coinage might be a DC 8-14.
* A small painting or sculpture might be a DC 10-16.
* A famous artifact or piece of art might be a DC 18-30+.
== Detection ==
A specialist in forensics has advantage or even expertise in detecting counterfeits. A complex, old or designed to be difficult to counterfeit item, may be much easier to examine than to create. A simple object may be harder to examine than to fake.
To discover a counterfeit with an Intelligence (Investigation) check could depend on how much you beat the DC by when you create the object. I use 10+difference to determine how hard the counterfeit is to detect.
If you have a gold piece of the local kingdom, and use that to create a counterfeit duplicate (DC 10, you roll a total of 19), the DC to detect the counterfeit is 10+9 (DC+(Roll Result-DC)) which results in a final DC of 19 to detect
An object that is found in the wrong location, or doesn't fit the observer's expectations can increase that character’s suspicion (and thus create favorable circumstances for the checker’s opposing Investigation check). Lighting and equipment used to investigate the piece in question also plays a great role in the difficulty - a note glanced over in torchlight by a grumpy guard in heavy rain outside a gate might have a harder time noticing discrepancies than the tinkering gnome in a well lit study room with his triple-enhancing monocle and a lifetime experience in forensics.
=== Types of Forgeries ===
Bureaucracies are rife with licenses, paperwork, deeds, writs, manifests, lists of every conceivable kind, travel passes, identification, and more. A skilled forger can move at will through these environments, and one based in an urban environment with a restrictive government can become wealthy very quickly selling documents to the underworld and the oppressed citizenry alike. 
Some common documents include the following:
* Identification: Since photography isn't available, handwriting is used to establish identity, and the state will often keep files of all its citizenry's signatures to check for people trying to commit fraud. 
* License: These can be anything from weapon licenses, to a merchant's right to trade, to a freelance hunter or mercenary operating in a foreign territory. 
* Military ID: Much like citizen's IDs, the military also requires paperwork for its armed forces. 
* Deed: Paperwork establishing ownership of property is done by nearly all governments, to prevent fraud and disputes. 
* Will: A final will & testament declares who is to own the deceased person's personal property upon their death. 
* Cargo Manifest: Merchants have to include a detailed list of goods being transported to ensure what is shipped arrives at its destination intact, and without being skimmed by the merchants.
* Certificate: Guild membership includes embossed, ornate paperwork showing the owner's inclusion in the Guild. These are almost always kept by the Guild itself. 
* Letter: Communication between people. A rogue can do a lot of damage with forged letters. 
* Bill of Sale: If you are in possession of valuable property you didn't pay for, you can forge a bill of sale to make it appear that you bought and paid for the property legally. 
* False Chart: You can falsify a chart in order to lead someone using it into dangerous waters or to hide the location of something you don't want that person to find. The navigator making use of your phony chart is entitled to a Knowledge (geography) check (DC equal to your Forgery result) to detect the chart's false nature; if the navigator's check fails, she accepts your chart as genuine (and takes a significant penalty on any course setting she attempts based on your false chart.
* Letter of Marque: A letter of marque is a document issued by a government that gives a private citizen the right to attack that governments enemies at sea and confiscate their property. It's often a thin veneer legalizing piracy against the ships of a particular kingdom or realm. Your fake letter of marque testifies that you have been given the right to attack certain ships - through often only in certain waters.
* Owner's Papers: Most civilized kingdoms require a ship to carry papers naming its owner, home port, trade taxes that've been payed off, or licenses for special goods, if any. You might find it convenient to draw up convincing facsimiles of these documents as you need them, instead of dealing with time-consuming and costly paperwork.
In places where travel, trade, and war at sea are common-place, governments naturally develop various licenses, letters, and documents in order to regulate and administer the activities of those who travel by sea. A skilled forger can be a valuable asset, especially for a captain or crew who wish to pass off a stolen ship as their own vessel.