Go over the floor with a hammer and nailset and knock down all the raised nailheads and other obstructions.
Load a rotating floor buffer with the roughest grit-level of circular screening pad. Run it over the whole floor, with the direction of the boards. It should take off the most of the top layer of gloss from the floor.
Go over the floor with a vacuum cleaner to take up the dust.
Re-load the buffer with a medium grit pad. Go over the floor again, in the same way as you did with the rough pad. It will remove the rest of the gloss. Vacuum.
Re-load the buffer with a fine grit screening pad. Go over the floor a third time, smoothing it out completely. Vacuum.
Gently stir a can of polyurethane. Don't shake it. Pour a puddle of it across the width of the floorboards in the corner of the room furthest from the entrance.
Set a floor gloss applicator in the gloss and gently pull it backward along the floorboards, with the direction of the planks. Pour more gloss as needed and continue pulling it across the planks. Make the puddles of gloss the same length as floor-gloss applicator. Apply gloss to the whole floor in this manner.
Let the gloss dry for a day. Sand it by hand with extra fine sandpaper, very gently, to de-gloss it enough to allow the next coat to adhere.
Lay down the next coat of polyurethane in the same manner as the first one. Let it dry, sand it, and lay a third and final layer. Let the final layer set for two days before walking on it.