Tongue and groove flooring that's installed with nails generally takes the nails in two different ways: you shoot nails through the surface of the first course to provide a solid starting base for the rest, then secure the rest by shooting the nails through the sides of the boards, thus hiding the heads from the surface. Make sure to let the boards acclimate to the room for a week or more before you lay them.
Roll out rosin paper underlayment over the whole floor, laying it in slightly overlapping courses and stapling it down.
Lay the first floor board alongside the wall where you want to start, with the grooved side facing the wall and spaced 3/8 inch from it. Shoot nails every 8 to 10 inches along both edges of the board.
Lay another board off the end of the first one, locking them together by their tongue and groove edging. Keep the second board 3/8 inch from the wall. Secure it by top-nailing, as before. Repeat and continue, laying the flooring along the whole edge of the floor. Cut the final piece on your mitre saw to fit.
Set the second course of boards alongside the first, locking the boards along their sides by the tongue and groove milling. Nail the second course by shooting nails down through the sides of the boards, positioning the finishing nail gun just above the tongue. This will allow the nails to be hidden by the next course. Shooting in nails at the tongue every 8 or 10 inches.
Continue across the room, installing the courses of boards by nailing through the sides above the tongue. Cut the ends as needed.
Cut the boards of the final course along their lengths, on a table saw, so they fit against the last wall with a 3/8-inch space. The spaces will be hidden when you install the floor trim.