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To mark out the radius it is best to use a solid item, with a nail through at the length of the radius. When you find the swinging point, line the nail up and knock it in. This gives you a stable method of marking out the arc very acurately. Marking out the arch on both sides of the wall, exactly the same each side, is crucial to good finish. Check all the measurement are precise before cutting. The arch bead is going to follow the cut edge. Make several vertical cuts with a saw, up to the arc line. Then carefully cut around the arc line. Break the pieces and run a knife through to finish the cut. Finish cutting out the balance accuracy is crucial. Screw timber blocks in place along the length of the flat section. Make sure the blocks are of the same width as the wall faming studs. Note that I have screwed the blocks 11mm in from the cut edge. The center strip should not drop below the edge of the sheet as the edge of the sheet will guide the arch bead. This arch is easy to construct with no framing Cut small blocks and screw into place, as shown around the arc. Note that I use a small block of plaster to make sure the board will not be lower than my cut edge. Keep the blocks small and spaced apart. Cut the strips of plaster slightly narrower than the opening and much longer in length.Wet both sides twice with a wet sponge, lean against a low object. As they get a good bend, lean them against something steeper. Be patient or they will break. Screw them up when they have bent roughly to the radius required. Take a lot of care, or they will break. Screw the bottom first, then work up to the top. Staple the arch bead at the bottom first, then push it hard into the bend and staple the top. The bead will follow your cut sheet without any flat spots. Provided it is positioned right, staple all the way around the radius. Mark the top with a chalk line to get a perfectly staight, level top. Staples need to be closer together than a normal external. Where ever possible do both curves and the flat section in one length If you have a wide arch and you need to join the angle, put the join at least a metre away from the curve. The end result is a perfectly balanced arch. Full arches are done exactly the same way.