Locate all the studs in the walls where you will install an L-shaped, prefabricated cabinet. Mark the locations of the studs on the walls with a pencil.
Hold one 2-by-4-inch piece of wood against one wall just below where an L-shaped cabinet will hang. Use a level to ensure the wood is horizontally straight. Drill screws through the 2-by-4-inch wood to attach it to the wall. Repeat the process on the other wall on which the L-shaped cabinet will hang. Place the second 2-by-4 at the same height as the first 2-by-4.
Hoist the prefabricated, L-shaped cabinet onto one the walls with the help of an assistant. Balance the bottom of the cabinet on the top of the 2-by-4-inch pieces of wood.
Ask your assistant to hold the cabinet in place. Attach the cabinet securely to the studs in both walls with the drill and screws. Repeat the procedure to install each L-shaped cabinet.