Carry the bevel to the miter saw and slide the blade around to match the angle of the bevel. Check the angle shown on the miter saw guide. Divide that number by two and slide the saw blade to that number. For example, if the angle of the bevel were 120 degrees, you would slide the saw blade to the 60-degree mark. If the wall is not square to the floor, you can use coped joints.
Coped joints are special for inside corners that make a tight, gap-free connection.
To cut a coped joint, start with an inside mitered cut. Then make a 90-degree cut along the margin between the finished wood and the bevel. You don't need to make a mitered cut on the trim piece that butts against your coped cut. Use a stud finder to locate and mark the wall studs. If you don't have a stud finder, you can locate the studs by tapping a nail through the wallboard in the area that will be covered by baseboard.