Now that you have a length and a width, the next step is to staple gun two pieces of elastic down the middle of the board. I'm going to be suspending 12 books here, so I used just over 14 feet of material for each side. Start each piece of elastic with the excess facing in. Staple this in place and fold the elastic back over to start running it down the board. Measure out 14 inches, staple it one inch farther and repeat until you get to the other end and fold the excess back inside again before stapling it.