Sand the floor with 80-grit sandpaper. You may want to rent a large rotary floor sander for this job; otherwise you can use a hand-held disc sander. Start at one corner and sand with the grain. Move slowly backwards until you reach the other corner. Turn the sander around and finish that corner. Using a hand-held disc sander, sand the corners and along the walls that the floor sander can’t reach. Vacuum the floor area thoroughly.
Change to 150-grit sandpaper and sand the surface again. Vacuum the area when you are finished. Sand the area one last time with 220-grit sandpaper. Vacuum the floor with a shop vac to get all the dust up. Be sure to get the dust out of the cracks before you coat the wood with stain.
With a cloth or brush, apply a clear pre-stain wood conditioner. This will give the stain a more even appearance instead of one that is blotched or streaked. Allow to dry 15 minutes, and then wipe the excess away with a clean, dry cloth.
Apply stain in the direction of the grain, with a brush or cloth. Work the stain into the wood. Work one section at a time. Stain an entire board without stopping in the middle. It is important that you work in sections and completing an entire board. This way, where you stop and start again will not show as much.
Wait 30 seconds to a minute, and then remove the excess stain with a clean, dry cloth. Check the color. Wipe in the direction of the wood grain. This will blend the light and dark areas together and give you a uniform color. Do not rub the wood too hard, and change rags when they become saturated with stain.
Allow to dry for 8 hours or overnight.
Brush on a second coat if you want a darker stain. For pine floorboards, it is usually best to stop at one coat for a light color. The wood floor is now ready for the polyurethane top coat to be applied.