1. Go into build/construction mode (B for desktop)
2. Select a constructed building room (home icon) (if there is work there it will prevent you selecting it until the work there is finished or cancelled)
3. Click the little plus symbol on the top left of the room popup menu (if you have no trinkets to equip it will do nothing (but should tell you you don't have available trinkets)
The radial menu:
I hope this helps, took me ages to find this silly thing.