Hold the base and top plates together with their ends aligned, then measure 15 1/4 in. from the end farthest from the door opening. Draw a line across the edge of the plates and mark an X right of the line. From here, mark a series of lines—one for each stud—spaced 16 in. apart, with an X to the right of each. Mark the plates to indicate a door opening.
Separate the plates and nail studs to the right of each line. Use two common 16d nails driven through the plate at the top and bottom of each stud.
Single-frame door openings require four pieces of lumber. Measure your door; then make the opening 2 in. higher and wider. To remove the sill plate in the opening, use an eight-point crosscut saw to cut almost through. (Protect the floor with masking tape.) Knock out the piece with a hammer and clean it up with a chisel.
Of course, I've never actually done it. I don't own an eight-point crosscut saw, you see.