What do I need to do to finish an old pine wood board floor?

You can rent a sander or hire someone to do it for you. If you like the color of the wood, you can go ahead any poly the floor. If you want to change the color, stain it first and then poly.

https://www.youtube.com/watch?v=TcGMtVXWNsA

U have what everyone wants. If they r heartpine u have a true treasure. Don’t funk them up with stain. They will darken with age WHERE there isn’t a rug. Any wood floor will darken where it is exposed to sun and light. Don’t use waterbase poly, get a pro to come in and do it perfectly. Be sure u hire a highly recommended company. No bubbles ever in poly.

Treasure them.

You can rent an oscillating sander (not the drum) from Home Depot. Get some 100 and 120 grit sandpaper for it and a buffing pad. Sand with the 100 grit first the 120 grit. Smooth out with the buffing pad. Put down at least three coats of a water based polyurethane. Dries fast and doesn't yellow.

You got it. If you would like the floors to be a little darker this would be the time to stain them too. Don't try to use a drum sander, takes lots of practice and can do lots of damage. My Home Depot rents two different vibrating floor sanders and they will do the job if the floor doesn't need much of the surface removed. The more you spend on the polyethylene floor finish the more durable it will be. You get to choose the gloss level on it from shiny to flat. Totally a matter of personal preference.

Sand, stain and poly it to maintain its new finish.