start with door or window frame to make first cuts
cut one piece of molding for each stretch of wall
Mark and Cut the Molding
At your miter saw, mark the molding for your first piece (Image 1). It's simplest to start with an inside corner (or a door or window frame) and use a square (90-degree angle) cut. Cut one piece of molding for each stretch of wall, remembering to use 45-degree angled cuts for the corners (Image 2).
If a wall is longer then the length of a single piece of molding, two pieces should be mitered together. Use opposite 45-degree cuts at the ends that meet to create the miter (connecting each length of molding with a 45-degree mitered edge will create a cleaner, smoother joint). Make sure you position the seam over a wall stud so that the two ends can be nailed into the wood. In most cases, wall studs are spaced 16 inches on center.