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.