I've not done this achievement myself but this might help ( not my advice, just what someone else has posted )
Returned the Thieves Guild to its former glory
In order to get this achievement you have to do enough side missions (and then a main mission for each of the 4 major cities) for the Thieves Guild, namely Delvin and Vex, so you can restore the Thieves Guild to its former glory. It’s basically
a matter of doing a set of mundane jobs throughout the cities of Skyrim until the guild catches someone’s eye there, then doing the main mission in that city. There’s a lot of tooing and froing to be done here, so don’t say you haven’t been warned – remember
as well, pick two missions up at a time (one from each Delvin and Vex) and then drop them off two at a time.
By my reckoning, you have to do around 40 smaller quests overall (well, that’s how many I did), as well as 4 bigger quests in the 4 major cities (not Riften) – these 4 missions will be given to you while you do the smaller quests at various intervals.
It’s 350 gold a mission to start with, 400 later on and they are quick and relatively easy (except the Bedlam stuff), but yes, it’s boring as sin!