After reading about Brunelleschi's Dome, my brother and I had an goal to build an arch out of snow. It doesn't follow his stone chain method, but it is a free standing arch.
Things you'll need, - Snow of the right consistency. Not too dry, not too wet. Packable. - Cool temperatures. This arch was built in warm temperature, which led to its short life of about an hour. - Support braces. We used a series of sawhorses, boards, and snow. - Ladder - Block mold. We used kitty litter buckets. It should have flat sides and have a slight taper. - Some sort of scrapping tool.
Find a flat surface and clear an area about 3'x3' down to the ground. For our arch we had a distance of about 14' from center of support to center of support. As you can see from the picture, I used an ice scrapper to measure an equal distance from a point we wanted as center. Also notice we used a larger form for the first two rows of the arch.