A leveled rifle
A correct cant on your rifle is critical for accurate shooting on medium and long distances. Even slight cant deviations of your rifle will change your point of impact of each shot. This effect is even more magnified by the hop-up which will cause the BBs to fly left or right depending on the position of your rifle.
Mounting the bubble level
The easiest way to mount the level properly is to align the reticle of your scope with an object that is already leveled with the ground.
A window for example. Align the reticle with the edge of the window and manually adjust the level until the bubble is perfectly in the center. Then tighten the screws equally on both sides and you are set.