A trampoline that tilts can throw the jumper off the side. The safest surface on which to erect a trampoline is a perfectly flat, level lawn; unfortunately, not very many lawns meet that criterion. But if your lawn slopes less than 1 inch for every 7 horizontal inches, you can correct for this by digging trenches into the up-slope legs. This can take some checking and rechecking, but it's worth it to have the safest possible trampoline setup.
Set the trampoline in the area in which you will use it. Set a plank across the middle of the trampoline, so that in runs straight up and down in the direction of the slope.
Stand on the down-slope side of the trampoline, and place the carpenter's level on the plank. Raise the end of the plank until the carpenter's level shows that the plank is level (the bubble is in the center of the two lines). Use the ruler to measure the distance between the top of the trampoline and the upper edge of the board. This is the total amount of slope you need to correct.
Divide the diameter of the trampoline (in inches) by the total amount of slope. If the result is greater than 7, the area has too much slope to be corrected for the trampoline; you should move the trampoline to a different location.
Move the trampoline out of the way and dig a trench under the area where the uphill legs of the trampoline will be placed. The depth of this trench should equal the total amount of slope you measured. Use the carpenter's level to make sure the trench is level.
Move the trampoline back to its original position, with the uphill legs in the trench. The trampoline will be somewhat unstable because the side legs will now be higher than the uphill and downhill legs. Dig trenches under the side legs until both the uphill and downhill legs are seated firmly.
Check that the trampoline is now level in all directions, and adjust the trenches if necessary.