Riana from Huntsville Ontario is struggling with getting the miter cuts right for crown molding.
First rule, use a large miter box (purchased, rented or home made) that allows you to make simple 45 degree with the molding standing up in its regular position. Well, almost in its regular position.The table of the miter box is the ceiling for the molding, so everything is up-side down.
Play with scrap molding until you have succeeded in cutting one good inside corner pair and one good outside corner pair of boards. Then label these boards as in the photo.
Always take these samples to the ceiling, check out exactly which piece you are about to cut in the real molding. Take that piece to the miter box and position it and the saw until the cut is perfect. Now substitute the sample piece with some real wood, and cut the proper angle on the end every time.
For a more complete overview including a mitre saw mitre/bevel angle chart, check out Cutting Crown Molding With A Mitre Saw. And you can also see my video on cutting crown molding in a power miter box.