If you have to ask the question, leave it to a professional. The ideal size drill will be the core diameter of the stud. Put a drill bit behind the stud until you can just see the drill behind the threads. Remove action from stock, make sure the fore end is deep enough to take the thread.leave 3 inches from ends of stock, use tape to protect finish and drill vertically! Tighten stud until holes are at 90 degrees to length, use spacer if it's not. I have found if you start to screw the stud in in the correct alignment, using downward pressure and a Tommy bar for leverage, it usually fits perfectly. Have messed up a couple of early attempts, so take care! David