Skyrim:Under New Management

Quick Walkthrough
# Attend the ceremony.
# Receive the Guild Master's Armor from Tonilia.
# Periodically, search the Guild Master's Tribute Chest for loot.

Detailed Walkthrough
Improving the Thieves Guild
Note: This section only describes prerequisites for this quest. The quest will not begin until you complete all prerequisites.

In order to restore the Thieves Guild to its former glory and assume the title of Guild Master, you must perform small jobs in four holds of Skyrim: Haafingar, the Reach, Whiterun, and Eastmarch. To get an improvement job, talk to Delvin Mallory or Vex; you can have one quest active from each. Once you have completed five jobs in a particular hold (except the Rift, where the Guild already has influence), Delvin will offer you a special "reputation" job to increase the Thieves Guild's influence there. You may start taking these jobs at any time after officially joining the guild (completing Taking Care of Business). Completing all four reputation jobs is a prerequisite to becoming the Guild Master.

You are assigned targets at random, without regard to the number of jobs that you've already taken in a particular hold.  You can check your Quests log to review how many jobs you've completed per hold. You can accept and quit the jobs at will, which can be used to your advantage if you are assigned jobs in holds that you have already completed, and if you wish to increase your efficiency by matching two jobs to the same hold. If one of Vex's jobs will take place in the same hold as Delvin's, such as a sweep job and a bedlam job both in Eastmarch, then the value of the objects assigned to be stolen in Vex's jobs also applies to targets in Delvins's Bedlam jobs, and both can be completed together. This is an easy way to steal two birds with one... theft. However, there are fewer jobs offered in the Reach (Markarth) than in the other three holds, so it's best to take any job there that you can get. All failed jobs will be listed in the completed quests list.

For each reputation job that you complete, the Thieves Guild's headquarters is incrementally improved. Each successful completion adds a merchant to the Ragged Flagon, and completing the third and fourth jobs adds recruits to the guild. Completing these jobs also increases the gold available to all of your fences up to a maximum of 4,000. In addition, gradual cosmetic improvements occur in the Ragged Flagon - Cistern; a carpet will be placed in the center platform in the middle of the room, and banners of the Thieves Guild will be emblazoned on the walls. Also, when you are arrested in a hold over which the Thieves Guild has influence, you will be given an option to bribe guards even if you don't have the Bribery skill perk.

The four "reputation" quests are activated when the quota is reached for jobs in a given hold:


Ceremony
Once you have completed all of the reputation quests and Darkness Returns, you will be told to talk to Brynjolf about becoming the new Guild Master. (If you do Darkness Returns before the Guild is fully restored, you will automatically get this quest after completing the last reputation quest.)

When you enter the Cistern, a crowd will be gathered, including the Thieves Guild regulars, Karliah, and Maven Black-Briar. After you speak to Brynjolf, there is a short ceremony, during which you are named Guild Master.

After the ceremony, you must again talk to Brynjolf, who will hand you the Tribute Chest Key and the Amulet of Articulation. The key unlocks the chest near the Guild Master's desk. The Guild periodically places loot in here as a tribute to you, and you are free to take it. Brynjolf also tells you to see Tonilia about getting your Guild Master's Armor set. The Guild Master's Armor set appears graphically similar to the set Mercer Frey wears, and is just a boosted version of your other Thieves Guild Armor.

Achievements
One achievement is unlocked at the end of this quest:
 

Notes
If you complete the last two special jobs at the same time and then become guild master, you can turn in for both special jobs and have Delvin compliment you twice, saying you're the best thief in the place, despite his assertion that he'll only ever tell you once. 
In order to complete this quest as quickly as possible, it is possible to immediately quit a job and ask for another in order to get one in a particular hold. There are no penalties from Delvin or Vex, however, avoid doing this with the Burglary, Fishing, and Heist jobs due to associated bugs. You can also make a save just before taking a job and reload it until you are given the desired hold; this avoids any negative consequences. 

Bugs
 
 Check that the quest "Taking Care of Business" (TG01) has completed. To make sure, use the console command "setstage TG01 200".
   Use the console command   to start the quest manually.
 
There are spare pieces of Thieves Guild Armor in The Ragged Flagon - Cistern, in the training room, with the locked chests. You can pick these up and trade them in for the upgraded versions, which also allows you to get the Guild Master's Armor and the achievement.
 
A possible fix is to run outside when finishing your first interaction with Brynjolf. You need to run outside as quick as possible by going to the ladder and exiting. You should see the quest update pop up reading: "Started Under New Management". Afterward, go back inside and proceed to Brynjolf. To talk to Brynjolf, let him go to the middle of the Cistern first, and wait.
Another possible fix is to get the quest about fixing debt from the guy at the Riften stables. The player will be told to talk to Sapphire. Instead, Sapphire should be sitting on the bridge just inside Riften and when approached will say "I don't have any business with you." or "We'll talk after the ceremony." Attack her 3 times, and this should cause her to start attacking the player notifying them that "Under New Management" has been completed and that the player should talk to Brynjolf. Upon entering The Ragged Flagon, the player will be required to pay 1000 gold to Vex as reparations for killing a fellow Thieves Guild member. After doing so, speak with Brynjolf and the questline should be completed.
 This can also be fixed with the console command  , which will start the quest chain if you have already done the main quests mentioned above.
 
 
 
 
 

Quest Stages