What is the best way to make a lap joint?
When you have a wall that is too long for one piece of crown, then select two or more lengths of crown that will cover the full length of the wall with enough excess to account for the joint cuts and the corner cuts (the larger the molding the more extra length you will need). Make the lap joint with the saw blade tilted at 45 degrees and the miter set to 30 degrees. For each joint you do, use the same setting for the blade tilt. In other words, cut both sides of the joint without moving the saw adjustments. Cut the first cut for the joint on one side of the blade and cut the second piece on the other side of the blade. Doing it this way will assure that when the two pieces come together on the wall that they will be straight. Always cut the lap joints first and then proceed with installing the crown. The first piece you install will not have to be measured, just installed. The second (or last) piece will have to be measured to length, then the corner joint cut and installed.
