First of all, you should take a measuring tape for taking the measurement of your doorway where you want to construct the arch. It will help you in deciding the size of the arch according to the width of doorway. Make sure you also decide the thickness of the arch structure which is generally made of plywood.
You should select ½ inch thickness of plywood and the same thickness of drywall. It will help in preparing the arch with equal size from all sides.
Now you need to measure the length of the lumber which should be between 2 to 4 inches. You need to rip three lengths of lumber and each length should be 8-foot long which is the standard size for an average arch.
After that, you should cut the ripped lumber and its size should be according to the length of circular saw. Then nail this piece flat to the horizontal side of your doorway. The size should accurately fit with the upper side of doorway.
Now you need to cut two more pieces of ripped lumber that should be equal to the width of the arch. Make sure you these pieces are according to the curve of your arch.
Then you should cut two rectangular pieces of plywood which should be ½ inch wide and should be equal to the length of horizontal pieces of arch framework. Then clamp the plywood with C-clamps that will make the plywood pieces together.
After that, you should draw the sign of an ellipse on the surface of top plywood sheet which will set up an arch shape. Take a jig saw and cut through both pieces of plywood in order to give it a shape of arch.
Now lift these pieces of plywood and attach it to the specific place of the doorway. Nail these pieces of plywood with 8D nails and cover the vertical and horizontal sides with drywall which should be ½ inch thick.