An arch is half a circle standing on two legs, which means the same blueprint that gives you a good circle gives you a good arch. Build the ring, keep the top half, throw the rest away.
The quickest approach
Decide how wide the opening should be — that width is your circle diameter. Generate the circle at that size, then read only the rows above the midpoint. Those rows are your arch profile, and they are already symmetrical, which is the thing that is nearly impossible to get right by eye.
Build one side, then mirror it block for block. Count outward from the centre column rather than inward from the edge; miscounts at the edge are much harder to spot.
Shallow arches and gateways
A semicircular arch looks Roman. For something flatter — a bridge span, a wide gateway, a viaduct — use an oval instead and take the top of that. A 40 wide by 16 tall ellipse gives you a long, low arch that looks structural rather than decorative.
Pointed gothic arches are two arcs that meet at a peak. Generate a circle a little wider than your opening, take one quarter of it for the left side, mirror it for the right, and let the two curves meet in the middle.
Depth makes it look real
A one-block-thick arch reads as a cutout. Give it two or three blocks of depth and it becomes architecture. The thickness setting in the generator handles this for you — set it to 2 or 3 and the blueprint thickens the whole curve evenly, which is far more consistent than adding a second ring by hand.
Stairs and slabs around the outer edge soften the stepping. Place the full blocks from the blueprint first, then go back and swap the outermost block of each step for a stair facing outward.
Get the blueprint
Start with the circle generator, set the width to your opening size, and read the top half. The block counter tells you how much material the whole arch needs before you start hauling it up scaffolding.