Locate the studs using a stud finder in the wall in which the cabinet will be located. Select two adjacent studs to build the cabinet between and mark in pencil the inside edges of the studs on the wall.
Draw two vertical lines along the wall in pencil, incorporating the marks you made along the inside edges of the two wall studs. Use a yardstick to keep your lines straight. The two lines should be equal in length, drawn to the desired height of your cabinet.
Connect the ends of the two parallel lines you just drew on the wall by drawing two horizontal lines. Use the yardstick to draw these lines straight. The vertical lines of your completed template delineate the sides of your cabinet and the horizontal lines will represent its top and bottom.
Use a drywall saw to cut along the template lines. Set aside the rectangular, drywall cutout once you have removed it from the wall.
Measure the width, depth and height of the space between the wall studs. These measurements will serve as the dimensions for your built-in cabinet.
Cut a sheet of 1/2-inch plywood using a circular saw to the width and height of the open space. Position this board, which will be the back panel of your cabinet, upright inside the wall space between the studs. Drive nails with a nail gun through the board to attach it.
Subtract 1/2 inch from the depth measurement. Cut two pieces of plywood for the top and bottom panels of your cabinet to using the depth measurement you calculated and the initial width measurement.
Slide the bottom panel horizontally into the wall space so it lays flat against the bottom. Drive nails from a nail gun through the bottom panel at an angle into the wall studs along the sides and the back panel along the back edge.
Position the top panel flat against the top of the wall space and attach as in Step 8.
Cut two sheets of plywood for the side panels of your cabinet. Two sides will be the height of the cabinet, minus 1 inch, and the same depth measurement you used for the top and bottom panels.
Sandwich the resulting two boards vertically between the ends of the top and bottom panels. Drive nails through the side panels into the wall studs.
Measure and mark the desired position for your cabinet shelves along the length of the side panels in pencil.
Cut your shelves to the depth and width of the cabinet, subtracting 1 inch from each measurement.
Install the shelves horizontally between the side panels, securing them in place with nails.
Paint or stain your completed cabinet. To ensure that your cabinet blends with the existing decor, consider painting it the same color as the wall or the same shade as the baseboards and trim in your room.