Minecraft wiki entry for t-gun, associated text: Place a button on the back of your cannon, connect it to the main TNT charge (here, powered from the left side) and all the aim TNT. The piece on the other side will be slightly tricky: you need another line of redstone to power it. Don't place it on the other side of trench, as it will be used later. Instead, place redstone on the ground. In-line with the mounting block, place a solid block, a piece of redstone on top of it, and on the solid block next to where the aim TNT will be.