Step 1: Basic Build
Step 2: Water & TNT Cannonball Thingy
Step 3: Light 'em Up
Light the TNT that was added in the basic build step then wait about 3 secs then light the TNT cannonball thingy.
Step 4: Boom
Make the cannon as long as you like, although the longer the cannon gets, the harder it is to light all the TNT before some explodes...