Seal first then stain or varnish. If you don’t seal the varnish and stain just seep in. You CAN keep doing it until it fills up but that is not the right way. Varnish is deceptive. It will look like it has filled and layered then when it dries nah nah nin nah nah. Start over. Do not pass Go and spend your $200.

First vacuum, then scrub the bare wood with a little soapy water and a brush. To remove any dark stained spots, try scrubbing with some oxalic acid (Ie: Dekswood made by Flood). Wipe off with a damp rag. Let the bare wood dry at least 48 hrs after cleaning. The cleaning might raise the grain slightly, requiring light hand sanding with #80 grit sandpaper or screen on a sanding block.

Unless you have an unfinished floor that was waxed only, then yours has flooring finish such as polyurethane, and it also might have a coat of stain. If it was stained, you will have to guess what stain was used and restain the cleaned wood and let it dry overnight before applying the first coat of finish. If it just has clear finish, then you probably can match the color tone of the old floor by applying a similar finish. You must take care to feather the edge of the newly coated area to minimize lap marks. Wait for the finish to dry thoroughly (at least overnight), then lightly sand by hand with #120 grit sandpaper or screen on a sanding block. Vacuum and tack the sawdust and apply a second coat of finish.