Let's say you haven't built a road to your property yet and you have to hike there and want to build a cabin that is level. One easy way to do this is to take a garden hose long enough to reach any two points you want to find the level for. Then buy some clear tubing from Ace Hardware or someplace like Home Depot or something like that and buy about 6 to 8 feet of clear tubing about the same diameter as your hose. Then you get some grey tape (duct tape) and tape the tubing that you cut in half on either end of your hose. At this point you have 3 to 4 feet of clear tubing on either end of your hose. Then (you are going to need a water source near or on your land to do this). You fill your hose with water.

If you have stakes you can drive a stake into the ground wherever you want to show level in your rectangular or round or square or whatever shape you pick house or building you want to build. Then with water in the hose enough to come up into the clear in two places (this is much easier with at least 2 people by the way. However, if you taped or tied one end you might do this alone too.

So, since water ALWAYS will find the level anywhere on earth if you see water in one clear end it is going to be level always at the other clear end as long as the water is still and not being moved around by anyone like a child or something.

YOU take and mark where level is on one stake and then walk over and mark where level is on any other stake you have driven into the ground and as long as no one messes with those stakes you have a level for your land where you want to build from then on.

Always trust the level of the water because it will find level between any two points on your land even if it appears to be an optical illusion on some pieces of property around the world.

I haven't watched any of these so I don't know if they are any good but here are some online videos regarding all this: