If your floor is coated with clear finish, it must be stripped with a sander before refinishing. You can rent a drum type floor sander and a hand-held disc sander; ask for a demonstration of their loading and use.
Clear the room of furnishings and seal it off from the rest of the house with plastic sheeting. Remove shoe molding from the bottom of the baseboard to permit working close to the walls. Pull out any staples left from carpeting. Repair damaged or loose floorboards; drive protruding nailheads well below the surface and fill the holes with wood putty. Vacuum, then inspect the floor once more.
Sand the floor first with the drum sander and 36-grit sandpaper. Raise the drum off the floor, turn on the power, then lower the drum carefully, keeping a firm grip on the handles. Work across the floor with the grain, then return to the beginning and strip the next section parallel to and slightly overlapping the first. Next sand along the walls with the hand sander and 36-grit paper.
Repeat the sanding process twice, using 80-grit sandpaper, then 100 grit paper. Use a scraper to strip the corners. Sand by hand at each end of the room to remove marks where the sander has stripped across the grain. Vacuum thoroughly between stages and wear socks or white-soled sneakers to prevent scuff marks.
Wipe the stripped floor with a tack cloth to remove dust. Apply clear sealer or a stain (preferably one combined with a sealer). When dry, buff with fine steel wool. Vacuum thoroughly. Apply polyurethane, shellac, or varnish, using a roller for the center area, a brush around the edges.
Allow to dry at least 2 days, then buff with steel wool again; vacuum. Apply two coats of paste wax, followed by a buffing with lamb’s wool. (Note: some polyurethanes do not have to be waxed. Check the label. With a no wax type, buff with steel wool and wax only if you prefer a softer patina.)