Easy, sort of.
You 'box' the archway forgetting about the arch part (for now.) Then, once you've got your WIDTH and HEIGHT all framed out, you cut your arch (on a jig-saw, or the like). You cut BOTH sides of the arch at one time (so you either mess them BOTH up at the same time, or you luck-out and cut them right at the same time.) You can nail them temporarily together until you're finished cutting them, makin sure you don't nail where you have to cut OR where they would get in the way of the saw.
Once you've got your sides cut (keeping your pairs together), cut a number of pieces of 2x4's that, when they're nailed BETWEEN each side, they're as wide as the arch! Use a piece of scrap wood to be sure you cut everything the right size. Once you've done that, you can use it as a template. Then, nail the 2 arches to the archway and buy some plastic/flexible corner bead, along w/some drywall, and finish the job!
If you have any particular ?'s, ask away. Just try to envision what's going on. (I kinda gave you the short version but I warned you it was 'Easy, kinda ...'
My best to ya and hope this helps.
Jay J -Moderator
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