Here is a sketch that may help explain the framing detail for a rounded corner or arch:
Cut out the desired curve from a piece of 3/4" plywood with a jig saw, etc. You need two such pieces for each side or four for both sides.
Nail or screw the drywall on the face of the wall first. Cut the shape of the drywall out with a drywall saw, following the plywood edge as your guide.
Depending on the radius of the curve you can either dampen the drywall that is 4 1/2" wide to bend over the plywood curves, or cut slits in the back of it, about 1" apart and bend it into the curve while screwing it in place. Try to get the drywall tight into the curve to get an even, balanced curve. It may not be perfectly smooth, this can be done with the mud later. For the corner bead, use plastic bead that is already slit around the sides and nail it in place, adjusting the curve in and out a bit to make it a smooth curve. The smoother the bead is the better the curve will look with the mud on it.
When applying the mud use a wide enough drywall knife to span both plastic beads and use them as a guide, 6" knife should do. When sanding use sandpaper without a holder so it will curve around nicely. Sanding sponges work well here, too.
The thing to watch is the accuracy of the plastic trim installation since these are the edges you will follow when applying the mud and sanding it off. They act as guides. Take the time to put the plastic bead on to form a nice curve, square across the wall with the bead on the other side.
Mud up the curve and wall as normal, at least two layers with a third as skim coat to touch up here and there.
Wow! A curved doorway that wasn't that hard after all.
Dave(Ask Dave) (About Dave)
Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.
Membership gives you full access to our hundreds of how-to articles, woodworking plans, converters, calculators and tables. Our Stair Calculator is one of the most popular on the internet. We have projects you can build for (and with) your kids, furniture for your wife, and sheds and gazebos. If you run into a problem or need advice your Membership includes unlimited email questions to me through our Ask Dave quick response button.