Plan your closet. Most closets are at least 2 feet deep. Measure the area you want your closet to be and find the studs in the wall using a stud finder. The walls need to be attached to an existing stud, so this can determine the size of your closet. Measure the frame for the pre-hung door. Measure the height from the ceiling to the floor and subtract 4 inches to find the height you need to cut your stud pieces. Decide on the number of shelves you want in your closet.
Use the circular saw to cut your wood. You will need to cut two 2-by-4s the length of each wall you are building for the top and bottom of the frame. You will need enough stud pieces that you can place one on each end of the wall frame and spaced 18 inches across the frame. You will need one piece cut to the width of your pre-hung door frame. Cut 2-by-4s to use as shelf supports in your closet. You will need one that runs the length of your closet and two that are the width of your closet minus 2 1/2 inches for each shelf. For a closet with three shelves you will need three pieces the length of the closet and six pieces the width of your closet. Cut plywood for each shelf. It should measure the length of your closet by the width of your closet.
Frame the walls of your closet. Place the top and bottom pieces on the ground. Nail a stud piece to the corner to form a square. Make sure the corners are square and nail in the stud pieces every 18 inches from one end to the other. You should use two nails per each side of the stud. Do not place any studs where the door will go. Install a stud on either side of where the door will go, so you can nail the frame for the pre-hung door in place. Nail in the cross piece for the top of the door at the correct height.
Put up the walls by nailing them into place along the stud you have marked on the wall. Start at the floor and nail every 18 inches up to the ceiling. Nail the frame into place along the floor on either side of each stud. Nail the top of the frame into place into the ceiling joists.
Sand and paint the 2-by-4s and plywood for the shelving system. You can stain the wood or paint it a contrasting color to the walls of your closet or the same color.
Screw up the drywall at each stud, along the top and the bottom. Add another row along the middle of the drywall to help it stay firmly in place. Use the drywall tape to close any gaps between the drywall pieces, and the drywall compound to fill in the seams and cover the screw hole. Sand the compound after it dries and paint the closet.
Install the shelves by nailing the 2-by-4s you cut for support into place so that the top of the 2-by-4 is where you want your shelf to go. You will need to nail the 2-by-4 into each stud behind the drywall. The long piece will go along the back of the closet, and the two shorter pieces will go on each of the shorter walls. Nail the plywood shelf into place at each corner and along the sides and back every 12 inches to secure the shelf. Install a clothes-hanging rod at the top of the closet.
Place the door into the frame you built. Use a level to make sure it is level. Check to see if the door will swing freely and use a mallet to put the frame into place. Nail it into place at each corner and every 18 inches along each side of the frame. Put trim around the door, and baseboards around the floor of the interior and exterior of the closet.