Cut all your boards. If you are using 3/4″ MDF or 3/4″ Plywood, have Home Depot of Lowes cut the plywood down into 5 – 48″ long strips, 11 1/2″ wide, and then 1 strip, 48″ long and 14″ wide, as shown below. Then cut the pieces to size at home using your miter saw. If Home Depot or Lowes does not do cuts in your area, you will need to cut as shown below. Just remember that the saw blade takes up 1/4″ and you cannot mark all the boards out and cut. You must measure the first board, cut, measure the second, cut, and so forth.