How do I properly paint a wood floor made of plywood?
if you use one of those powerful floor sanders, be sure to use fine grit paper and go easy.  Plywood can damage easily and those sanders are aggressive and designed for hardwood. A good 6 or 7 inch DA sander would be a good bet, especially around the edges. Be careful, don't tear up the plywood with a monster sander!!!  lol.

As far as paint, select a good grade floor enamel. Roll it on with a short nap roller. A moehair or 1/4" nap is perfect for leaving a good smooth finish without a lot of orange peel effect.