The promotion to Footpad requires you to perform one assignment, Untaxing the Poor, for Armand. The quest won’t be accessible, however, until you have sold fifty gold worth of stolen goods to Ongar in Bruma. At that point, return to the Garden of Dareloth and visit with Armand. He will ask you to steal some collected taxes and tax records from the South Watchtower.
The first floor is a public area. Enter between 9:00 AM and 11:00 AM and there should be no guards. Go up the ladder to the second level, take the ladder on the left to go to the third floor, and then the next ladder on your left to access Lex’s private quarters. If you’re seen on the second or third level, you’ll be asked to leave, but if they catch you in Lex’s quarters, you’ll be arrested on sight, so make sure that you’re sneaking or using chameleon or invisibility spells. The desk with the records is immediately on your right when you enter Lex’s room. Steal the records and the gold and leave the Watchtower the same way that you entered. Return to Armand at midnight and give him the records. The achievement will unlock when the conversation with Armand is over.