Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.
First, and most importantly, go outside. This experiment is very messy.
Pour enough water into the canister so that it is about 1/3 full.
Break one of the Alka-Seltzer tablets in half.
Place the Alka-Seltzer into the canister, close the lid, and step back a safe distance (3-4 meters).
Wait a few seconds... and then watch your film canister rocket go bananas!