Step 1
Mark the dimensions of your rough opening on the existing wall frame. Your width should be 32 inches to allow an inch on each side for shimming (and insulation, if you are framing an exterior door). Your height should also be an inch taller than the door, but subtract 1.5 inches to factor in the sole plate, which you will saw out later to let the door extend to the floor. Mark your width on both the top and sole plates (horizontal boards at the top and bottom of the frame) and the height on any studs within or next to the rough opening.
Step 2
On the top and sole plates, mark spaces for two adjacent studs (each 1.5 inches wide) beyond each edge of the 32-inch width you marked in step 1. Each side of your doorframe will consist of a full-size king stud (in the outermost space) and a trimmer stud cut to the height of the door (in the space nearest the doorway).
Step 3
As necessary, saw through and remove any studs that fall within the rough opening you marked in step 1.
Step 4
Cut two 2-by-4 studs to match the other studs in the wall frame, and put them in the outside spaces you marked in step 2. Nail diagonally through each stud at the top and bottom to anchor it to the wall frame.
Step 5
Cut two 2-by-4 studs to the height you marked in step 1, and put them in the inside spaces you marked in step 2. Nail each stud to the top and sole plates, and to the king studs you installed in step 4.
Step 6
Cut a 2-by-4 stud to the width of the space between the king studs (which should be 35 inches). Rest this header stud on the trimmer studs you installed in step 5, and nail it securely in place.
Step 7
Cut a 2-by-4 stud to fill the gap between the top plate and header, and set it midway between the ends of the header. Nail it to the top plate and header.
Step 8
If it hasn't already been removed, cut out the sole plate between the trimmer studs you installed in step 5. Your frame is now complete, and you should be all set to install, shim and trim your new 30-inch door.