Step 2: Stack and Fix
As you begin filling your forms and making your blocks, try and make them as compact as possible. Any weak areas of the block will eventually make your arch shift, trust me. Let the natural taper of your form begin to slowly form an arch. Do this by stacking every block with the small side of the taper towards the center of the arch. You will see it slowly bend towards the middle.
As the arch builds it's important to step back and adjust the pillars. It's snow, so it will shift and compact. Make sure you help the snow pillars get back to a true state, perfectly straight up and down from left to right. Obviously, keep the arch arching, but the front to back may need to be straightened.