Run a one-shot or up to a 3 session game. Have your Newbie GM sit with you. Let the other players know that this is going to be a walkthrough. For a walkthrough, everyone at the table should be explaining the choices they're making and why they're making them, and feel free to ask questions as you go.
"Ok, so notice how I'm having the goblins run in and attack now? This
is because they heard the fight from a few rounds ago, and I figure
they need a little bit to grab their stuff. Also I'm having them
attack from range, because they figure if the heroes survived the
first fight, they might be tough."
"I'm ending the combat now, even though there's 3 guys left, because they all have like 2 hitpoints, so we know this is pretty much done. No point in playing out the rest."
If you can't do a full session, consider a few scenes - run a combat, run a social scene, run an exploration, point out why you're doing what you're doing and what makes it work.