I used Microsoft Visio many times to try and visualize how all of this fit together and to check dimensions and angles. This is an end-on diagram of the ceiling beams and molding build up around them.

When you are selecting moldings a catalog like this one: http://www.mouldingandmillwork.com/pdf/IND3829-Chicago%20ProfileGuide.pdf

is extremely helpful in deciding on what molding shapes to use. I had to go to our full service lumberyard to get all of my molding. The home improvement stores don't carry the variety, and especially the lengths I needed.

The beams in the living room are almost 16 feet long. I was able to find the WM 54 molding at the lumberyard in 16 foot lengths so I didn't have to splice the crown halfway up the side of a beam. One less joint to open and to make a splice like that is always a hassle.

Key to parts in the Picture (remember to click on the picture to enlarge it)

Brown is the original beam 1 3/4 x 4 3/8
Red is the 1/4 Inch MDF cover we machined
Blue is WM 54 - 2 1/4 x 9/16 Pine Crown molding
White is our Baseboard Wm 620 - 4 1/4 x 9/16 flipped upside down.
Green is the WM 320 - 4 1/4 x 9/16 crown molding on the wall