Step 1. Find a supply of snow.
Step 2. Using your hands, form snow into a rough spherical shape, compacting it with gentle force to ensure structural integrity and ease of delivery.
Step 3. When satisfied with the either the form or function of your creation, biff it at the nearest person.
Step 4. Run.

Finally, note that because the snowball traditionally disintegrates on impact, perfecting the size and shape tends to be sacrificed in favor of speedy mass production. This is a metaphor about alchemy--OH GOD THEY'RE GAINING ON YOU, RUN!