Decorate your index card which will form the body of the rocket.
Roll your index card into a tube (width wise for a shorter tube) and then put the film canister inside with the open side of the canister facing out one end of the paper tube. Tape the paper tube onto the film canister. Once that is secure you can tape closed the long seam on your paper tube. Try to minimize overhang of the index card from the film canister.
Cut a few small triangles for fins. Tape them to the bottom (film canister side) of your rocket. You can experiment with the number and shape of fins to see what makes your rocket fly best. You can also make a paper cone for the nose of your rocket if you wish.
Hold your rocket upside down and fill the film canister with water.
Quickly add half of an Alka-Seltzer tablet and snap the lid onto the film canister.
Place your rocket right-side-up on the ground, stand back, and watch it launch!