TESV: Skyrim – Restoring the Thieve’s Guild to its former glory
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.