Use a water level by aligning each end of a water-filled tube with whatever object or surface you are leveling. Some water levels are electronic and come with a hose attachment, whereas others utilize a simple tube and water design. A water level's use depends on its type.
Fill the level with water leaving a couple of inches empty at each end, and then tap out any air bubbles. Attach both ends of the tube to what you are leveling. Make sure the tubing is not kinked, twisted or flat. Make a reference mark on the object you are leveling, and mark the same on the other end.
Professional Deck Builder provides a guide to making your own water level and how to use it correctly.