Cutting inside corners is not easy but can be done to look professional with just a few tips and patience! Your first step of course is getting your measurements and purchasing your materials. When buying your materials buy a little extra because the leftover pieces are not damaged they can always be returned.
Cutting your cope first is usually best that way if something goes wrong with the other cut the piece of molding is still long enough to be re-cut. Generally, you will not have to make a cope cut on both ends of the molding if you layout the design right. If you do have to make two cope cuts on the same piece it will probably be the last piece put up. Always do your longest pieces first and save the scraps to make cuts on for templates.
Most wall and ceiling joints are uneven. This happens because of the build-up of paint, stucco, drywall tape, and mud. It is especially uneven sometimes at the corners where critical measurements are needed for the inside molding cuts. You could find a difference of a sixteenth of an inch to as much as a half of an inch.
This will give you a less perfect look when you put your molding up. With this in mind, you will want to make sure your top and bottom edges meet before the inside edges otherwise there will be a gap. Always snap a chalk line around the room. This way you will be able to see where your trouble areas are ahead of time.
Another tip for making your job easier is to apply a backing trim to the walls and placing your crown molding on that. It adds another dimension to the finished look and also makes applying your molding easier. The back trim piece will be screwed into the studs of the wall giving you a good solid surface to nail against when applying your molding.
One of the main reasons for a poor look to crown molding is nails not hitting a solid surface to hold the molding firmly. You can buy flat pieces of trim with decorative edges to coincide with the appearance of your crown molding.
The easiest way to cut the inside corner is to turn the molding upside down so the edge that will go against the ceiling is flat to your workbench. Double-check your measurement and make an inside miter cut. It also is helpful to make the cut a little long.
This way if an adjustment is needed you can take a chisel, sandpaper, or file and take a little off at a time. Once you have made your miter cut leave the piece right where it is and move only the scrap piece out of the way. Take a look at your wall and envision how the other piece will fit with this one.
Now, take a scrap piece and make your cope cut on it and see how the joint fits the wall. If all went well you will have a perfect fit and a feeling of a good job done!