Gather materials: TNT, dispensers, redstone dust, redstone repeaters, buttons or levers, water buckets, building blocks, and a flint and steel or ignition source
Build a base platform with your chosen building blocks
Place a water channel or water-filled trench at the front where the TNT will launch
Place dispensers facing into the water channel
Load the dispensers with TNT
Place redstone dust behind the dispensers
Add repeaters if you need to delay the firing sequence
Connect the redstone to a button or lever
Place a block or barrier at the back to contain the setup
Test-fire the cannon
Adjust repeater timing, dispenser placement, and water length for better range
Add more dispensers for a larger cannon
Keep the firing area clear of flammable blocks
Reload TNT into the dispensers before each shot