Place the arch casing up where it goes, then trace inside and outside of the casing putting marks on the wall and jamb about where it meets the straight casing.
Then place the straight casing in its position and mark them. Now you know where the two pieces intersect. Put the arc piece back up and mark the intersection points on the casing, then take to saw and align the blade with your marks on the casing, after you get one side correct do the same for the other. Then cut the straight pieces to fit.
Its called a Hunting Miter Angle.
You can also swing the arch on a pcs of plywood laying down on saw horses then intersect the bisecting angles. Sometimes this is much easier
If the window is a half circle and the casing was made correct there should be no bisecting miter unless you want to bevel the joints together. If its a eyebrow,elliptical arch then that's another story