Hi,
The instructions that come with the GTX II trigger explain everything very clearly. A couple of screwdrivers, a Philip's head of the appropriate size and a small standard screwdriver, are the only tools required. Included in the instructions is a picture that shows the relationship of the varius parts. Aside from the old trigger, the return spring, the return spring retaining pin, and the tiny adjustment screw that sets inside the rear of the trigger housing are not needed with the new GTX II trigger.
After you remove the stock, just compare the trigger mechanism with the instruction sheet and all will be very clear.
Da Tuna says it only takes 10 minutes, it took me about that long, perhaps a little bit longer, although I didn't put a timer on it.
Bevo66.