How does one unlock them?

There are 3 main sources of skins

1. Zombie skins - these can be unlocked by getting infected during the Survival event in Mid-February until the start of March.
2. When the special Human Zapper opens. This usually opens during Morostide, Mid-October until End of October, though it has opened at other times as well.
3. Misc skins - some come from special events, certain achievements and other rewards.

Was there a particular skin you were interested in, or just skins in general?

The blob kitty/caterkiller skin. I'm assuming these ones are achievement-based?

Blob kitty is a zapper skin, so should be available soon. Once its announced on the news, you go to the zapper in Darkside and there will be a button for you to click to stand in front of the zapper, I can't remember but it costs some amount of sP and wizard tokens. You then get randomly zapped with a human skin. So no guarantee you'll get the blob kitty. There are options you can pay more sP for and limits how often you can take your chances. It usually runs for a week or two so you'll have lots of chances to get skins.

Caterkiller skin is a zombie skin, so not available until Feb.

thanks!