Measure the width and length of the ceiling with a measuring tape. This determines the length of the coffered ceiling beams.
Locate the ceiling joists with a stud finder. Mark the location of each ceiling joist with a pencil. You need to line up the coffered ceiling beams so that they cross or run directly beneath ceiling joists.
Make a mark on the ceiling at each end of the wall, directly below a ceiling joist, at 16-inch intervals. Snap a chalk line on the ceiling between each pair of marks. Mark and snap lines at each end of the remaining two walls to create a 16-by-16-inch grid.
Cut sections of 1-by-4-inch lumber to the length of the ceiling with a circular saw. You need one lumber section for each 16 inches of ceiling length.
Place a sturdy step ladder at each end of the room. Have a partner stand on each ladder and center one section on the chalk line crossing the ceiling joists nearest one end of the room. Using a drill and a screwdriver bit, insert a 4-inch wood screw through the beam into each ceiling joist. Repeat this process for each parallel chalk line.
Cut 12-inch sections of 1-by-4-inch lumber to cover the perpendicular chalk lines between the installed beams. Have a partner stand on a ladder and place one of the sections along a chalk line at one end of the room. Attach the beam to the ceiling joist with 4-inch wood screws. Repeat this process until you have completed the coffered ceiling.
Fill in the exposed screw heads with wood putty, using a 1-inch putty knife. Stain or paint the coffered ceiling beams as desired.