It's easy, in a messy, dusty, laborious sort of way! Really, good prep is key. Clean it well, sand it carefully, wipe down with a tack cloth (no dust allowed), stain and seal. But, remember that stain is very watery and can leak onto anything in the vicinity. So think through your plan. If you can pop the trim off the walls and stain it elsewhere (like the garage) and then pop it back on, that's what I would do. But that's me.