Test-fit the trim by holding it in place. Check to be sure you've cut to the right length. Trim as necessary.

Once the length is correct, attach the trim to the wall with nails. Since using a hammer can result in damage to the wall or molding, you should use a pneumatic nail gun to attach the molding. Remember to wear proper eye protection while operating nail guns. On air-powered nailers, uou'll also notice a powerful blast of air is driven out of the back of the gun each time you use it. This is completely normal.

Using the nailer, continue to attach the remaining trim pieces.