By "underside," I am primarily meaning the "special" area, which should be trimmed for obvious hygenic reasons -- those nifty yellow accents.
Lay your dog down, roll him onto his back, and carefully proceed to trim away the messy hair surrounding the genital area.
This will minimize the dog soiling himself and it will also help to keep him cool on warmer days.
If your confidence allows, cut down the hairs extending from the tip of his willy.
Otherwise, don't bother, as you may well circumsize him.
Just let that be a semi-annual groomer's task.