Sweep all debris off the floor and examine carefully. Tighten loose boards by face nailing with size 6D or 8D flooring nails, preferably into the joists. Countersink the nails well below the wood surface with a nail set. Then get down on your hands and knees, and examine the flooring for any exposed nails or nails not driven well below the wood surface.
Replace any damaged or defective flooring boards. Use a plunge router or rotary tool such as the RotoZip to cut across the ends of the boards that must be removed. Make sure you stagger the lengths of any joining boards that must be removed.
Cut the replacement board to the correct length and cut off the lower edge of the grooved side. This allows you to push the tongue of the new board into the groove of the old board, and drop the overlapping edge down in place on the tongue of the other joining board.
Fasten in place with glue and face-nail with size 6D or 8D flooring nails. Set the nails well below the wood surface. Use prepared wood putty to fill any cracks between the newly installed and old boards and to fill over the nail heads.
Remove the base shoe around the entire room using a wood wedge behind a pry bar to protect the baseboard from damage by the pry bar. If the base molding and base shoe have been painted many times, you may be better off replacing it. If necessary the base shoe can be left in place, but this will require more careful edge finishing.
Most flooring, especially hardwoods such as oak, are 3/4-inch thick when they are laid. They can usually be sanded and refinished several times. Thinner, 3/8- or 1/2-inch-thick floors should be approached with caution. Repeated sanding can wear down the joining edge and expose nails. If the flooring is laminated, professional help is necessary. If unsure of the floor's thickness, remove a floor-heating register or the shoe mold and baseboard to expose an edge of the flooring so it can be measured.