FG1,
To answer your question, your hands and eyesight need to be pretty good and mine aren't necessarily the best but I manage. You have to locate the position of the sear arm first. You do this by holding the lock in position above the stock. You need to hold the lock parallel to the stock surface and look straight down on it to make sure the orientation relative to the touch hole, web, and wrist is correct. One you have it right, mark where the sear arm touches the stock. Some folk give the lock a good whack with a mallet over the sear arm, or you could just use a pencil to mark the location. If you use a pencil, then you must make sure that you mark around the bottom of the sear arm. It is essential that you get the location of the sear arm on the stock correct. There is a little be of latitude after you drill the hole, but not much.
Next, you drill a 1/4" hole into the stock where the bottom of the hole touches the bottom of the mark for the sear arm. Double check to make sure that the hole you drill is fully within the lockplate once inlet. Also, make sure you drill the hole to it's full depth once the lock is fully inlet. Usually this will be a little more than halfway through the stock.
Once you have this hole drilled in the proper location, the rest is pretty easy. You just keep removing wood that touches the lock until it is in place. You will use a lot of marking compound. I use magic markers. You are going to have to use measurement and experience to remove most of the wood before close fitting is necessary. Once you have the lock in enough to mark screw locations, I suggest that you drill holes for them to the finished depth. They form reference marks. The same can go for the mainspring.
Mark E.
