Step 1: Materials
optional: glue gun
Step 2: Measure it out
Measuring out my books, the average width was one inch and the ideal length of elastic turned out to be 14 inches.
Step 3: Staple the elastic to the board
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.