Another technique is to align the punch and tap it in to get the corner points to dig in. Remove the punch and mark an "X", corner to corner to determine the center. Drill press with proper Forstner bit and stop set to depth, center the bit and drill.
You can then reset the punch in the corner notches and whack away. Chisel out the waste but leave the flat created by the drill.
I also glue since the plugs can get bumped or loose as things shift from humidity. Usually just a dab of epoxy applied with a toothpick to the sides and flat. Not too much or it'll squeeze out