To become the Guild Master you will need to complete the main Thieves Guild questline as well as the four Guild special jobs. The special jobs are obtained by completing five normal jobs in the four main cities: Markarth, Solitude, Whiterun, and Windhelm. They can be any job, for example you can do three numbers job and two shill jobs and you still get the special job for that city. After you've completed both of these tasks you will be told to talk to Brynjolf about becoming the new Guild Master. There is a short ceremony in the middle of the Guild Hall, and Brynjolf will hand you a Tribute Chest key which 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. You are also told to see Tonilia about getting your Guild Master's Armor set. The Guild Master Armor set appears graphically similar to the set Mercer Frey wears, and is just a boosted version of your other thieves guild armor. -uesp
After performing five total Small Jobs in a city, Delvin will offer that city's special job. It may be less time-consuimg to quit jobs in cities in which you have already completed the Special Job. In order to restore the Thieves' Guild to its former glory and assume the title of Guildmaster, you must perform these four special jobs. Each Special Job will add a new merchant to the http://elderscrolls.wikia.com/wiki/Ragged_Flagon and add 1000 to the total gold of all fences in Skyrim, up to the maximum of 4000. Once all special jobs are complete, speak to http://elderscrolls.wikia.com/wiki/Brynjolf to be officially named Guild Master. - wiki