Determine the length of each wall where you want to hang your pictures, and have the molding cut a foot longer than that measurement at the home-improvement store. The extra foot is necessary if you have to turn a corner; it allows for the miter. To miter, measure the exact length and mark the wood. Hold the wood in the miter box, lining up your mark with the 45-degree slot, and cut. Paint your molding to match your trim.