Step 1: What You need
All you need is a half-block, any kind of whole block, a redstone repeater, redstone, a leaver/button. T.N.T and a bucket of water.
Step 2: The Base
You need to have a seven block line, and one next to it and joining it on with a block at the end.
Step 3: Stairs
Add stairs one block away from the conjoining block, and keep adding on.
Step 4: The Half-Block
Put the half-block at the end of the 7 blocks.
Step 5: Adding the water
Put the water where you started the stairs, it should stop at the half block.
Step 6: Adding Redstone
Put the redstone on the seven blocks as shown.
Step 7: Redstone Repeaters
Put the repeaters in the gaps on the right.
Step 8: Leaver
Add the leaver at the bottom where you connected the redstone.
Step 9: T.N.T
Add tnt from the stairs to the end, to finish!