Hi Ben,
good luck with the first build. I use an orbital sander and sand from 180 grit (120 grit if there are deep machine marks) up to 240 or 320 grit. Then I go over and hand sand with a sanding block 320 and 400 grit is usually enough.
Be careful to keep the orbital sander flat, replace worn paper discs and keep it moving a steady speed and don't stay in one place too long