Cutting inside and outside corners with a basic miter saw (no bevel
angle)is the easiest way to cut your corners. I would recommend
cutting your corners this way even if your saw has a bevel on it.
As long as your molding is not to wide.

You are going to hold molding in the saw on the same angle it is
going to be on the wall. By cutting like this all you need to do is
adjust the miter angle from left to right.

Take a small piece of your molding and hold it in the saw upside
down so that top and bottom of the molding sit flat against the
table and fence of the saw.

Imagine the fence of the saw as the wall and the table as the
ceiling. Now take a piece scrap wood and clamp or hot glue it to
the table of the saw as a guide. Now you know when you place your
molding in the saw it is sitting at the proper angle every time.

Sincerely,

Brian Kiernan