Setting Up on an Uneven Tripod Site
The primary purpose of a tripod is to keep a camera steady and level; sometimes it's easy to accomplish this goal because the site is flat. Other times are more challenging because you need to set your tripod site on uneven, rough land. Regardless of where you need to set up, you can have a level camera on almost any site when using a tripod.
Most tripods have a leveling bubble to help you out. The camera is level when the bubble lines up inside the small circle. If you're using a tripod that does not have a level, then you need to get a torpedo level. They're small levels that you can find in any hardware store. If using a torpedo level, you need to place it flush against the top of the tripod. The goal is to make the center bubble even.
Tripods have adjustable legs which makes leveling them fast and easy. Simply find your shot, set the tripod, and adjust the legs up or down until the bubble is centered.
Shooting in the Mud
If you're shooting in a muddy area where a tripod leg can sink slowly, then you need to figure out a way to compensate for this. One solution would be to lay a four feet by four feet piece of plywood on the tripod site to keep any one leg from sinking into the mud.
Sometimes the best shot needs to have the camera set in the most uneven place possible. The ground is so uneven that you cannot adjust for level using just the legs because the gap is too big. Don't let logistics discourage you; rise up to the challenge. You could rent what the the film industry uses to help them in these tight spots and get some apple boxes.
Apple boxes come in four sizes: Full (12 in. x 20 in. x 8 in.), Half (12 in. x 20 in. x 4 in.), quarter (12 in. x 20 in. x 2 in.), and eighth aka pancakes (12 in. x 20 in. x 1 in.). Thanks to their various sizes, you can stack them to fill in any gaps caused by uneven terrain. You also need to rent some wedges and shims (small pieces of wood) to fill in gaps of less than 1 in.
All you need for setting a tripod on an uneven site is the tripod, a level, and some apple boxes for extreme cases.