First, remove the trim. We tried to save as much as we could, to reuse when we finish the basement. Our woodwork is easily replaced, so I'm not sure it's entirely worth the effort and having to save a bunch of boards in an already small basement, but oh well.
(This picture doesn't entirely capture how bad that green was.
We have since painted it, as a temporary solution.)
Then, select straight studs. For this small opening, we needed five total. We measured the opening and cut the boards accordingly. We couldn't get the 16" on center spread, so we spaced them pretty evenly- I forget the exact measurement. We laid everything out, then nailed it in place (two nails at each junction).
We used regular drywall, although looking back I might have selected a specific product that our plastered recommended. I forget the name, but will try to track it down. It's a type of drywall that works best with plaster. But it's the same idea, and our regular drywall worked fine.We used drywall screws roughly every foot down the length of each stud.
Apply a plaster bonding agent to the drywall, and use a lightweight plaster. Spread it over the wall. We liked a product our plastered used- it sets in 20 minutes. That means you've got to work fast and will likely have a big a wasted material, but it sets much faster than some other products we used. Everyone has their own favorites, but we're happy with it.
So there it is! In a nut shell, that's how we removed the doorway, with tremendous help from an experienced neighbor and our plasterer. While I mostly watched the framing process, it's absolutely doable.