Step 6: Air Lock
This step can be done earlier but I chose to do it during the boil. In any case you'll want to keep one eye on the boil at all times. Did I warn you about boiling over?
I used a commercial air lock but you could fashion an air lock from a piece of clear vinyl tubing, one end fitted to the cap and the other end in a cup of water. The commercial air locks are only $1 or so and are included in the starter kits. I drilled a 3/8" hole in the water bottle cap. The air lock fits snugly in the hole. The idea is to let carbon dioxide produced during fermentation escape while keeping air outside from getting in.