To mark the positions of the holes, I insert a clearence drill into the hole in the butt plate, holding the plate firmly in place, and with a light hammer, tap the drill to impart a mark on the wood. Then I rotate the drill bit a quarter turn and tap again. What it makes is a + sign on the wood from the chisel end of the drill bit, exactly in the centre of the hole in the plate. With a sharp prick punch, I move the mark forward about 1/32" so that the pilot drill bit will make a hole that will draw the plate tightly against the wood. For a pilot drill bit, I use a 3/32 bit, and I drill down the depth of the screw. I eye-ball this operation. Once the pilot drill hole is made, I use a drill bit that will provide good purchase of the screw's threads. I use wood screws, preferring #10's for butt plates.