First measure where the molding will go. Molding typically comes in 8-foot (2.4-meter) lengths; however, some is available in 10- or 12-foot (3- or 3.7-meter) lengths, so you'll need to choose your molding before you can determine how much you'll need.
It's important to round when measuring. For molding that will go around the room's perimeter, round all measurements up to the next largest foot to allow for cutting and trimming of the pieces. For example, if one wall measures 8'3", round up to 9 feet (2.7 meters) when making your calculations [source: Royal Mouldings].
Be as precise as possible when measuring, and then divide the total by the length of the molding and add 10 percent more to the total to account for bad cuts [source: Stimpson].
Measure each corner wall angle to help calculate the correct miter angle. Some walls meet in unusual angles. It isn't unusual for supposedly perpendicular wall angles to be off as much as 3 degrees. While this may not seem like a big deal, if it's not taken into account before mitering the molding pieces, you can end up with a big gap between the pieces and have to make the cut again.
Before cutting, measure twice and test-cut the angles on scrap pieces of molding. When working on corners, measure the length needed to cut the shortest side of the molding. The top of the molding will be shorter for an inside corner; the bottom of the molding will be shorter for an outside corner. Measure again before making the final cut [source: Louisiana Pacific].
If two pieces are fairly close to fitting together, acrylic caulk can be used to fill the gap. Paint can even help make the gap disappear [source: Anderson].
Sometimes the trickiest part is cutting molding on an angle. Find out how on the next page.