Things you need (if you don't have'm at home) are:
-A MP5 (for this project I bought one if these toyguns on the Internet. They look like a MP5 and when you push the trigger they make and sound and there is an LED lightning inside the muzzle.
-Cables, connectors, buttons (I needed 5 of them because the MP5 came with a triggerbutton and two buttons for the foregripp flashlight), soldering-iron and tools like caliper, screwdriver.
-Some plastic plates (I bought them in a local DIY-store)
This is what I did with the MP5 I found. If you have a diffrent one it could be possible that you need to make diffrent modifications.
I also want to apologize if my english is too bad, but I life in germany and I'm not a native english speaker! ;-)
Step 1: Taking appart the Wiimote and the MP5
get a drill and drill them down until you can open the Wiimote. I'm going to say this only once:
The Wiimote is a sensitive device. Don't hit the board with the drill and be keep in mind that the screws are going to get hot, so that the plastic may melt, which will make things more complicated.
After opening put all the parts falling apart in a zip-bag, we will need them later.
Same with the MP5: unscrew all screws. I made a little sketch which shows where which screw is because they all had different lengths. There was also a ring on the muzzle I had to cut away and the flashlight, because it was glued. Looks like the boys over there in china wanted to make sure that nobody is copying there stuff. =) Remove all the electric stuff in there but be careful with the trigger unit. I used it instead of constructing a new button there.