Using Compound Miter Saw
Are you cutting it flat, or are you cutting it in position (upside down and backwards). Cutting in position is easiest for me. The fence is like the wall, the base is like the ceiling. You lay the crown upside down against the fence (thin edge up, with the profile facing you) There will be a void behind the crown as it sits on your miter saw. Crown stops are a nice accessory that help hold the crown in a consistant position. For a 90 degree corner, just cut a 45 on it while holding it in postion.
Practice with small scraps until you figure out what you're doing. Cutting crown is a little puzzling at first. Do an internet search if you don't have a how-to book on it. There's plenty of web sites that explain it in pictures if that's what it takes.