Side-to side leveling is the most important aspect of leveling a pinball
machine. If you have a level, just place the level on top of the playfield
glass at the front lock-down bar and raise or lower the threaded leg
levelers on the bottom of the legs. Then move the level to the back end of
the playfield glass and level it. You may want to double-check the front
again to ensure that leveling the back didn't alter the front.
If you don't have a level, then just eye the back box against the wall. It
will give you a good indication of level. Another method is to roll a ball
down the playfield and see if it rolls straight. If your playfield is
warped, then rolling the ball down will not help.