Use a stud finder to locate wall studs. Measure your cubbies and mark the wall with a pencil at the location where you want your cubbies to be installed, making sure there will be enough room to accommodate the height and width of the cubbies on the wall and that at least one side of each cubby will be mounted on a wall stud.
Hold the first cubby up to the wall at the spot you marked. Use a level to be sure it is straight.
Drill through the back of the cubby and into the wall at each of the four corners. At least two of the holes should be going into a wall stud. Make sure the drilled holes are slightly smaller than the width of the screws you are using to hold the cubbies in place.
Set washers over the holes, then screw in 2-inch screws.
Test the cubby to make sure it is secure.
Repeat Steps 2 through 5 to mount any remaining cubbies.