Make sure that all nail heads are hammered down and replace any boards that are rotten. Cover the floor with 30-pound felt paper and staple it down. This creates a stable base for the flooring, gives it a bit of cushion, and helps prevent squeaks.
From the wall, measure one board-width and snap a chalk-line. Lock the boards into place, then face-nail the first row of boards, then use a pneumatic flooring nailer to nail each nail into the tongue of the board. Space nails evenly.
Use a circular saw to rip the last board to fit snugly. Use a rubber mallet and a chisel to pry the tongue and groove tightly together. Face-nail the last board.
Set the trim in place and nail it in at an angle. Cut the boards to fit door openings, then cover any remaining boards with trim.