Gather TNT, dispensers, redstone dust, redstone repeaters, buttons or levers, water buckets, building blocks, and a target block
Build a long base platform with a wall at the back
Place a dispenser at the back facing forward for the TNT launcher
Place another dispenser or a TNT loading spot for the projectile TNT
Put water in the front section to stop the cannon from breaking
Add a line of building blocks around the water to contain it
Place redstone dust from the trigger to the dispenser
Add repeaters if you need a delay between shots
Load TNT into the dispenser
Place a button or lever to activate the cannon
Test the cannon and adjust the redstone delay and TNT amount
Add more dispensers and repeaters for a stronger shot
Use obsidian or other blast-resistant blocks for a more durable cannon