The casing , mulls , stool and aprin can be done in 3/4 ” lumber cut to proportion. The stool can alsi be done in 1“. The outside corner can be done in outside corner or a backband.

Sometimes the outside corner needs to be cut down on the side edge that meets the wall in order to lay flat.

1 x 4 Flat Stock and a 1-1/8" Pine Backband. Easy peasey.

It looks like there might be a bead or bevel at the inner edge like this:

There may indeed be a bead on the inside edge of the mouldings. If that's the case, I'd use a beaded chair rail backer and an applied backband. Wide, one-piece casings with the same details are less forgiving of imperfections in walls, jambs, etc. and tend to produce more "colorful" language on the job site.

Got it. Thank you all so much!

1 x 4 Flat Stock and a 1-1/8" Pine Backband. Easy peasey.

That looks more like a 1 x 6. It's basically a craftsman type look.

There may indeed be a bead on the inside edge of the mouldings.

I don't think there's a bead. I think it's just the trim on the inside edge of the window. Similar to below. BTW: these are 1 x 6

It's admittedly tough to tell from photos--particularly photos from different homes, but the apron under the kitchen windows looks to have a bead. It reads Greek Revival to me.

^^^ Ah I was looking at the edge of the trim around the window.

The three window photos I posted are from my house.