How do you get a froslass

First of all, you need a female snorunt.
In Emerald, Ruby, and Sapphire you can catch them in Shoal Cave during low tide.

In Diamond and Pearl, use the poke radar on routes 216 and 217, or at the Acuity Lakefront.

In Platinum you go to route 217, or at the Acuity Lakefront. or Lake Acuity at Night.

In Heartgold and SoulSilver, you get one from the pokewalker from the Icy Mountain Rd.

Then obtain a Shiny stone and use it on your female snorunt.
DO NOT EVOLVE SNORUNT INTO GLALIE