Here is the easy and cheap way. You will need a padded vise or sand bags. Have the rifle scope mounts installed. using a cheap hardware store line level, about three inches long and with a flat bottom. place it on the top of the flat part of the scope mounts and level it from left to right while the gun is secured in the vise or bags. without moving the gun, install scope and place the line level on the flat part of the top turrent and turn scope until it is level and tighten ring screws. recheck after everthing is tighten down. The most critical part is that the rifle is not moved, causing you to loose it's level, while mounting the scope. This is while I prefer to use a padded vise to hold the gun in place.
RELH