[Touch Pro2, Tilt 2 Windows Mobile General] redpoint73:
All you need to flash a custom ROM is on the ROM Development subforum for the TP2:

http://xdaforums.com/forumdisplay.php?f=490

The resources pinned to the top of the subforum are extremely helpful, and indispensable.

One of them is an excellent step-by-step guide for flashing a custom ROM, written for beginners:

http://xdaforums.com/showthread.php?t=550540

As indicated in the guide, you will need to HardSPL the phone. This unlocks SPL (secondary program loader), which is normally locked so you can only flash "official" ROMs from your carrier. HardSPL is here:

http://xdaforums.com/showthread.php?t=550131

Also, optional, but highly recommended (I do it before EVERY new ROM flash) is running Task29. This formats the ROM area of the memory, and completely wipes it, so no old fragment of previous ROM linger to screw up the new ROM. It only takes a couple minutes to perform, and can possibly save you the headache of some random bugs.

http://xdaforums.com/showthread.php?t=550131

Flashing is really not hard. Just read as much as you can on the threads I linked, and follow the instructions carefully. The threads are really long, and you don't have to read the whole thing, but read as much as you have patience for. Any question or issue you can possibly have about the process has probably been encountered by someone else already, and a solution/answer already posted. The more you know before you start, the less you will panic when something doesn't go exactly as you expected.

Keep in mind that flashing a custom ROM will void any warranty you may still have. Although, people generally have luck re-flashing the stock ROM, and re-locking SPL before returning for warranty service, so the repair people actually have no idea the ROM was ever modified. Also, keep in mind that there is a slight possibility of "bricking" or disabling your phone. But follow the instructions and be careful, and it won't happen. Main things to remember, is never mess with the phone (power off, unplug) while its flashing, make sure you have at least 50% battery at each step, and flash the correct software (flashing a CDMA ROM to a GSM phone, or ROM for a completely different phone are highly probably ways to brick it). But normally, it is very safe. People on here have flashed custom ROMs onto their phone dozens, maybe even hundreds of times, without ever bricking their phones.

There is no correct answer to the question "what is the best ROM". Its like asking "what clothes are best to wear", there is no correct answer, it depends on your personal tastes and needs. The best ROM for you depends on how much you want looks and functions over speed and stability, what software you want packed-in, or if you want a minimum of built-in software (a "lightweight" ROM), and to install your desired software yourself. Look through the various ROMs to see the features of each one, look at replies from users, which thread are most popular, and pick for yourself. If you still need some suggestions, Energy, simplicity, Jackos, and Valkyrie are popular. But it would be an injustice to other chefs to suggest that other ROMs are not just as great. If you scan through the ROM Development subforum, you will see some threads along the lines of "what ROM is best, fastest, etc.", and you can look at those for more suggestions/opinions.

Also be sure to read through the first several posts on the ROM thread you pick to flash, as there are often specific instructions and FAQs for that ROM posted by the chef.

You don't need to backup your old ROM. If you need to return to the stock ROM for some reason, just go to shipped-roms.com and download it:

http://shipped-roms.com/

You will however, lose your personal data (contacts, texts, email) when you flash the ROM. There are a bunch of ways to back these up. Microsoft My Phone, synching to your PC on Outlook, or if you are a Gmail person, using Google Synch, are all popular methods.