Making sticker packs is one of the ways to help out at the FSF office. This page contains the instructions for the task.
The FSF store sells variety sticker packs. It's handy to have these packed up and ready to go when they're ordered. You can find individual stickers in the shop inventory closet - this is locked most of the time and someone will have to let you in. There are also stickers in boxes in the sticker closet. Finished envelopes go in a cardboard box in the inventory closet. Every envelope should contain these stickers, while supplies last: