Remove the bolt from the gun, position the gun in some type of cradle gettin the barrel relativly level with the earth.
Find a small level, I have a 2" level in a small alluminum housing, Lay the level across the bolt raceway just to the rear of the rear reciever ring. Make sure the level is perpendicular to the actions center line. Level the gun with this level and hold the gun level with the cradle. Level your reticle to a plumb line on a nearby wall, or somthing similar while keeping the gun level. Tighten down and recheck everything, Then level your scope tube anti cant device, while keeping all the preleveled things level.
walla, level, level, level.