Simply unscrew the little black cap, slide the leader into the slotted stem all the way to the bottom and replace the cap.
These are a big improvement since the thingamabobber came out. They do not slide up and down on your leader and they don't kink your line.
Rarely does a product come along that is so far superior to all others, but the Air Lock IS vastly better than other indicators. Once you try these, you'll probably toss the others out. We did!
Small is approximately 3/4 inch diameter, Large is approximately 1 inch diameter.