To install crown molding, mark the studs, sand the rough areas, attach the corner blocks, cut and attach the straight sections, and fill the gaps. You need a stud finder, a hammer, brad nails, a ladder, sandpaper, a putty knife, a saw, measuring tape and a miter box.
Use a stepladder and stud finder to locate all of the wall studs, and mark each stud with a piece of masking tape. Inspect the walls for rough spots and other imperfections, and buff them with fine sandpaper.
Position a corner block so it is flat against the wall and ceiling. Nail the block in place, and repeat with the rest of the corners. Measure the distance between the corner blocks on each wall, add 1/8 inch, and record the measurement. Mark a section of crown molding to that size. Repeat with the other walls.
Clamp one of the cut molding pieces into a miter box with one end at the 90-degree slot. Saw through the molding, unclamp it, and set it aside. Repeat with the other pieces.
Grasp one end of a molding section, and have an assistant take the opposite end. Hold the molding up to the wall, and secure it with brad nails driven through the studs. Mount the remaining molding with the same technique.
Inspect the molding for gaps between adjacent pieces, and fill them with wood putty. If you find gaps between the molding and the ceiling or wall, seal them with caulk.