I bought 1, 4'×8' sheet of MDF a 1/2 inch thick at Home Depot for $20, and had them cut the sheets into 4 inch wide, 8ft long strips for me.
I should have listened to Decorchick! when she recommended painting your wall white or whatever color you would be painting it before you added the boards, but of course I didn't.
My husband cut seven (7) of the twelve boards down to approximately 66" and lightly sanded the boards to smooth out the rough edges before he nailed them to the wall.
My husband nailed each board 2 ft. apart starting from the middle of the wall....I know, I know we're not "exacto" on this one, we picked the middle of the wall due to our control units for the alarm and HVAC unit.
Then we figured out where we wanted the smaller pieces to fit to make the “boxes.” We chose a smaller row up top and slightly larger on bottom, and big rectangles in the middle. My husband used our hand saw to cut all of the pieces of MDF to fit in between the vertical boards.
And when everything was all nailed up, all we had to do was caulk all of the seams and spackle the nail holes. But my husband didn't listen to me and he covered them alright!!! With a huge amount of compound. I told him to use paintable caulk, which would have done the job without all of the sanding.
After all of the unnecessary sanding was finished it was time to paint! [and if you have already read "Wooda", Shoulda, Coulda you would know that because of my husbands excessive use of compound I had to paint twice...yes DOUBLE work for me!] I painted about 4 coats of Clark + Kensington Paint - Ultra White [that was FREE from Pleasants Hardware]. I used a brush to get into all of the nooks and crannies and a foam roller for the walls and boards.
And yes the MDF does stick out from the baseboards and existing chair rail a bit because it's a 1/2 inch thick, but I surprisingly like the way it turned out and for $25 it's been one of the cheapest projects that has had the most impact on a room!