Assemble a beam sample out of the materials you will use for your ceiling beams. Often these include a 2-by-4-inch board mounted to the ceiling and 1-inch boards for the sides.
Measure the full outside width of your beam sample. In most coffered ceiling designs a beam is mounted along each wall.
Measure the width of the ceiling and deduct the full outside width of your beam sample. Divide this amount by the number of recesses you want across the width of the room. Measure the length of the ceiling and deduct the full outside width of your beam sample. Divide this amount by the number of recesses you want across the length of the room.
Transfer your measurements to half-inch graph paper. Mark the full width of the room with every half-inch on the paper equal to 1-foot in the room. Mark the full length.
Measure away from the left wall side using the measurement of your beam width. Make a mark labeled X. Measure from X using the divided width measurement. Mark each section to the right wall side. You should end up exactly at the right wall side. Measure away from the right wall side to the left using the measurement of your beam width. Make a mark. Move to your left to your next mark. Measure away from the mark to the left using your beam width. Repeat for each mark until you reach the left wall. Your last beam is already marked.
Measure away from the top wall side using the measurement of your beam width. Make a mark labeled Y. Measure from Y using the divided length measurement. Mark each section to the bottom wall side. You should end up exactly at the bottom wall side. Measure away from the bottom wall side to the top using the measurement of your beam width. Make a mark. Move upward to your next mark. Measure away from the mark to the top using your beam width. Repeat for each mark until you reach the top wall. Your last beam is already marked.
Draw out your lines so you can see the beam and recessed areas clearly. These will look the best when the recessed area measurements are similar.