It is a rudimentary job to paint over plywood, but the key to painting over any bare wood is to prepare the surface. If the plywood is properly prepared, you can avoid problems down the line like discolouration. You also get a better finish than if you just slap the paint on the plywood as it is. Make sure the plywood is clean and dry before you begin.
Scrape the plywood with the scraper to remove loose debris.
Apply wood filler to pocks, holes or gaps to fill them.
Sand the plywood to your desired finish. The smoother the sanding job, the smoother the paint finish, although the paint will level somewhat as it is applied.
Prime knot holes and other discoloured areas first, with the brush, and allow the primer to dry. A little bit of extra primer on these areas will help control discolouration and help hide the imperfections.
Apply the primer. You can use a brush or roller. Apply the paint in the direction of the grain. Wait until the primer dries before proceeding.
Apply the paint using a brush or roller in the same way you applied the primer. When dry, apply a second coat.