What's the best way to go about painting subfloor?

If you are going to keep the painted sub floor as your floor I would use porch and patio floor paint. Behr, Glidden, Sherwin Williams are good brands. It will be more durable. I would cut in around the edges with a brush and then use a roller. You might want to prime first.

There are special floor paints available.

Use Porch and Floor Paint. I would use a wood filler to fill in any cracks, knots or gouges first, sand lightly, If it has knots with resin, then I would use some Zinseer B-I-N on those areas, then paint it. You can also do a stenciled design in two colors. I have also seen some great painted rugs on Pinterest using painters tape and various stencils. https://www.pinterest.com/search/pins/?q=how%20to%20paint%20rugs%20on%20floor&rs=typed&term_meta[]=how%7Ctyped&term_meta[]=to%7Ctyped&term_meta[]=paint%7Ctyped&term_meta[]=rugs%7Ctyped&term_meta[]=on%7Ctyped&term_meta[]=floor%7Ctyped

If you want a high shine gloss floor you could also epoxy the floor, you can buy Rustoleum Garage Floor kits in solids, flake, granite even glitter. https://www.youtube.com/results?search_query=how+to+do+floor+with+Rustoleum+Garage+Floor+epoxy+kit

https://www.youtube.com/watch?v=o2sVR-3BmJg

here is a gorgeous one.... https://www.youtube.com/watch?v=CZgAAmQqj94 they wear spike sandals over shoes to prevent marks