Clean the floor with a high-quality hardwood floor cleaner. It is important to remove all old wax or grunge or the new finish won't adhere properly. In some cases, a good cleaning may be all the floor needs.
Spread wood filler into all gaps that have developed in the floor. Use a filler that matches the color of the floor and sand it when it dries with an orbital sander and medium-grade sandpaper.
Screen the floor with a floor buffer and a 120-grit sanding screen. This will scuff the surface and prepare it for the new finish. Vacuum the floor when you are done and wipe it with a damp cloth to tack off all the sanding dust.
Spread a line of finish along the wall furthest from the exit and in the same direction as the flooring boards. Soak a long-handled finish applicator in the finish and, keeping the applicator slanted so that the finish trails off the end farthest from the wall, drag it along the wall to the end of the room.
Keep the applicator in contact with the floor and make a 180-degree turn and drag back the other way, overlapping about half the width of the applicator. Continue in this way until the floor is covered, and then let the finish dry for the recommended length of time. If you use a water-based polyurethane, this will be about two hours. For an oil-based finish, it will be overnight.
Screen the floor, wipe off the dust and apply a second coat of finish in the same way.
Screen the final coat with a very fine screen, 320 or 400 grit, and then mop on a coat of floor polish. Buff this up with a lambswool buffer for a final finish.