Note: Almost all steps are accompanied with images.
Image of a finished bookcase below.
Step 2: Optional Step: Tape Bottom of Box
Step 3: Step 1: Cut Flaps Off Box
B) Repeat step A) 3 more times.
Step 4: Step 2: Make and Insert Shelf
B) Staple them together.
C) Fold the longer flaps in half. (Groove the middle of the flaps to help fold.)
D) Put the longer flaps around the two connected shorter flaps as shown in accompanying image.
E) Staple them together.
F) Insert shelf into box.
G) Staple sides of shelf to box.