I came up with this idea, because my area is on a burn ban, so I couldn't do fireworks. So I went to play Minecraft, then I got the idea, I tried multiple cannons, and then came up with this. It's not to complicated. Just so you know, I DID use a mod. Too Many Items 1.7-1.7.2 by Marglyph.
Step 1: Supplies
The supplies you need are:
A flat area that is atleast 11x11
2 stacks of any non-flamable material.
4 buckets of water
1 bucket of lava
2 pressure plates
and ALOT of TNT.