Rules for designing a new spell appear in Street Magic, pp. 158-162.
A couple of excerpts:
A shamanic formula may be an item such as a painting, a carving, a set of runes, or a medicine bag. Hermetic formulae are typically represented as written/digital texts or complex diagrams composed of many arcance symbols. The formulae contain the same concepts but express them in different ways.... Any character who possesses the Arcana skill (see p.24) can design a spell formula.... Spell design is handled as an Arcana + Logic Extended Test. The threshold and interval are determined by the spell category, as noted on the Spell Design Table. (SM 158)
A Combat spell has a Threshold of 12, and a 3-month Interval. There are various conditional modifiers in the table that may apply.
If you copy existing spell stats, I think the cost should come out to be the same. You will want to use the 'Ice' elemental effect (p. 164):
Attacks with the Ice effect cover the target with a slick coating of frozen water. Treat the Ice effect as Cold damage (p. 154, SR4), except that objects or terrain affected will be encrusted with ice. Anyone trying to cross an icy surface may need to succeed in an Agility + Reaction Test to avoid slipping (with a threshold equal to the attack's net hits); vehicles must make a Vehicle Test (p. 159, SR4). Depending on the local temperature, ice may melt quickly. (SM 164)
The formula may exist on the Matrix or elsewhere; that would be at the GM's discretion.