How to Use a Spirit Level
Clean the level, removing all buildup and dirt from the edges.
Mark a line along the bottom edge on the wall.
Filp the level over so that the bottom becomes the top. Put the new top edge along the marked line. If the bubble is centered, your level is accurate. If not, it is defective.
Place the level on the surface of the object for which you want to find the true horizontal(the "horizon"). Make sure the spirit tube runs parallel to the object. Allow the bubble to float to the top of the spirit tube.
Put your eyes at level with the spirit tube. In order to get an accurate reading, close one eye.
Take note of where the bubble is inside the spirit tube. If it's centered between the lines on the tube, your object is level. If the bubble is to the right of the lines, your object slopes downward right-to-left. If the bubble is to the left of the lines, your object slopes downward left-to-right.
To find the true vertival or "plumb", repeat the same process vertically.