Crown molding is molding that is placed between the ceiling and wall. Crown molding is generally decorative, but can also be installed to hide flaws. This type of molding can meet on the inside or the outside of a corner. Angles for each of these are different, but they are cut on the same principle. Crown molding can also be joined along an expansive wall where no corners are involved.
Inside corners can be cut two ways. You can cut a 45-degree corner by holding the bottom edge of the molding (the wall side of the molding) against the bottom back edge of the saw base and back plate, then adjusting the base of the saw to cut at a 45-degree angle. Remember that the crown molding is getting installed on the ceiling. Therefore, the back plate on the saw is acting as your wall at ceiling height. The second piece of molding will need to be cut with the opposite angle.
Inside corners can also be cut by holding the crown molding flat against the saw, and adjusting the angles to 31.62 degrees and 33.9 degrees. (Most miter saws will have these locations marked on the saw.)
Turn on the saw and carefully cut through the piece of molding.
Place the molding flat against the back plate on the saw and set it at a 45-degree angle to the inside on the first piece of molding.
Turn on the saw and carefully make the cut.
Place the molding flat against the back plate on the saw and set it at a 45-degree angle to the outside for the second piece. Make the cut as you did with the first piece.