I installed one onto my WE M4, all that is required is drilling a small hole.
On the right side of the grip, there is a spring that pushes a pin up into the fire selector. you must drill a hole down into your grip(the MOE).
Be careful how deep you go.
I kept checking, and it wasn't deep enough. The next thing I know, It's too deep, and I had to shim the spring higher.
Imageshack made me register my email... so I removed the link... F***ers