Welcome to my first one! Here is how to make a safe-ish boom cannon that will send you flying!!
Step 1: Add the Outline
Make a 7x9 out of obsidian or Gold
Step 2: Tnt and the Stair Thing
Add TNT along the inside of the 7x9 then add a three block stair case on one of the longer sides
Step 3: Add Redstone
Put redstone on top of the 7x9 all the way around put some on the stairs as shown in the picture.
Step 4: Other Stuff
add two fences in the middle then put a bottom on the side of the top brick on the stairs.
Step 5: Boooom
Stand on the fence and press the bottom BOOOOM!!