Step 1: Construction
First find a nice place to build.
Then select obsidian and create a 7x5 rectangle on the ground.
Next on both sides go 4 blocks up from the base.
Now build a wall and a roof to create a 3x3 square in the middle. Then go to the front and place one block down in the middle on the bottom row.
Afterwords go to the back and fill it in except for one block in the center.
Create a hole by placing one block on each side. This will be the primer area.
Step 2: Loading and Firing
First select tnt and place one block on the stub in the front of the cannon. Then go two blocks more into the cannon and fill it from there. There should be a two space area in the front. Go to the back and make sure the primer is full. Select flint and steel and light the primer.
Step 3: Aftermath
As you can see there is minimal damage in the back and massive damage in the front i hope you enjoyed leave a comment below