Many people who want to install or replace wood trim, often referred to as molding, lack the skill to effectively accomplish the task since it’s not something they do everyday. A few simple tips will lessen the frustration and turn out a finished product you can be proud of.

Joints

The most common joint a do-it-yourselfer will make is a 45-degree miter. The minimum number of tools you would need is a combination square and a saw, but it’s worth the investment to buy a miter box to use as a guide. The simplest miter box is a molded plastic form with one slot for right-angle cuts two slots for 45-degree miter cuts. If you plan on doing a lot of trimming you should invest in a good power miter saw. Always read the instructions before using a power miter, and one of the most important steps is squaring the saw. This is done by placing a square between the fence and the saw blade to make sure that the saw is true. Even when the saw comes from the factory it may not cut an exact angle of 90 degrees; this would affect all the angles you attempt to cut. Each saw comes with detailed instructions for squaring the tool at the beginning of the manual. Whether cutting joints with a handsaw or power saw, lubricate saw blades with candle wax, beeswax, or furniture polish for easier cutting.

You should try cutting a few practice pieces before you work on full-length stock to get the hang of using whichever saw you feel most comfortable with. Holding the two pieces together to form a miter along the door or window jamb or wall opening will be the best way to see how you did. Sometimes the opening is not a true right-angle so you may need to check the corners for square and adjust your miter angles accordingly. Sometimes on drywall openings, the corners bulge out due to the corner bead underneath the spackle or the amount of spackle used on the finish. This is the worst place for irregularities to occur, since you have a joint installed directly over it. One way to correct this is to back-cut your miter joints. You can do this several ways.  The easiest way is to use a utility knife and cut the side of the casing that will touch the other piece on a slight angle so that only the face of the trim is full. Professional carpenters do this by tilting the blade of their power miter (compound miter) slightly, by a degree or two, on every cut.  A small block plane is another good tool; even coarse sandpaper will do the job. As long as you don’t alter the surface of the casing, you will still be showing a true 45-degree miter.

For baseboard corners that are out of square, you can achieve a close fit for miter joints by cutting one piece to exactly 45 degrees and then making minor adjustments to the saw to "sneak up" on the matching angle for the other piece. Use a scrap piece to find the other angle, once you have it, the saw is already set up correctly; you just have to cut it to the proper length.

  Some walls are not a true 90-degree corner and miter angles need to be adjusted

For the inside corner of baseboard, leave one end square in the corner and cope the other end. Coping is done by first cutting a miter on the base molding and then removing the exposed wood with a coping saw to match the profile of the other piece. Again, be sure to angle your saw blade slightly to back-cut the piece. This makes for a tighter joint that won’t open up when you nail it to the wall since the coped base will slide along the face of the square piece.

 Coping the base after a miter cut

 Checking the profile against the square piece of baseboard

Nailing

If you don’t have a nail gun, tap the head of the nail (the blunt end) into the face of the molding to create a depression; this also flattens the point of the nail slightly. Then simply turn the nail around and hammer it in as you would normally. This prevents the trim from splitting and is very effective on the edge of thin moldings like casings, for windows and doors. It isn’t necessary in baseboard, since that is nailed through the thicker face. Most professions recommend pre-drilling to prevent splitting, but this is only required near the ends of the trim or with hardwood moldings such as Oak, Maple, Cherry, and Poplar.