When you arrive at the police station, you will be unable to access the archives because the sketch artist is inside having a break down or something. In order to get inside, you will need to obtain a sketch of the thief. This is difficult to do as you will need to figure out where to go and deal with the first of many ambiguous quest objectives in this game. If you are having trouble with this quest, the guide below should do a lot to help you obtain a sketch of the thief.
Finding Mrs Nini’s Atelier
When you go inside and speak to her, you will suggest trying on some clothes in order to make yourself look like the thief so that you can sketch it easily. This will require you to go into the back room and try on some clothes and get some feedback from Mrs Nini to see how close to the thief you look. This takes a lot of trying but if you want to skip all of this, move to the section below.
Selecting The Correct Outfit To Match The Thief
Each time you select a new combination of items, Mrs Nini will give you some feedback on an item that is wrong to help you correct the mistakes. If you want to get it right the first time, dress up to look like the character in the image below.
While you are in the back room, you will also unlock a side quest to dress up as a guy in the picture. You can check out the side quest guide for A True Master of Disguise here.