Decide which way you want to hang the wall cubby, and find the center of each opening along the topmost vertical edge. Secure a screw in the center of each opening. For example, if you have a two-by-three cubby unit and you want to hang the unit going longwise, you'll have three screws along the top edge of the unit.
Find the studs along your wall with a stud finder, and make light marks on the wall with a pencil. It is important to secure the cubbies to the wall studs since the unit itself is heavy and you'll be adding more weight with toys and books.
Attach a picture hanging system, following manufacturer's instructions, to the wall where you want the cubbies.
Apply small rubber bumpers or felt circles to the corners of the wall unit to help protect the wall from the sharp edges of the cubby unit.
Hang the cubbies on the wall using the screws and the picture hanging system, being careful not to scratch or dent the wall as you place it.
Use a level to ensure the cubby unit is on the wall straight, and adjust as necessary.