It is easiest to finish sanding the doors before you apply trim, although sometimes, things get done backwards!
Sanding the Plywood
To sand wood, always start with a grade of sandpaper slightly finer than the existing wood finish and work up to a final finish grade. As Baltic birch plywood has a smooth planed surface, you will probably be able to start with a 320 grit sandpaper and move to a 600 grit to finish. Always sand carefully in the direction of the grain, never sand across the grain. Don't use a mechanical sander for final sanding! Change sandpaper when the sawdust fills the paper or when the grit wears off. When using fine sandpaper, knock your sanding block on a hard surface once in a while to clear the sawdust from the paper before continuing to sand. Use your hand to check all sanded surfaces to make sure they are smooth and even.
When you have finished sanding a piece with the first grade of sandpaper, proceed to finer (600 grit) sandpaper and carefully repeat the process.
If you will be using a water based (latex) varnish finish or stain, use a damp cloth to wipe the wood clear of sawdust. The water will also cause the surface grain of the wood to swell slightly. Re sand after the wood has dried, to take off the now swollen grain of the wood. Use a tack cloth (a commercial tack cloth, or a cloth you have put a bit of your final finish on and allowed to become tacky) to wipe the sawdust gently off the wood.
If you will be applying a wood stain, apply it now according to the directions on the stain. Sand the wood after applying your stain with a finish sandpaper, and use a tack cloth to clear the sawdust.