How to install a non workshop mod into a steam version of the game?

[crude cloud]

(Concluded) Never tried this before, but im curious on putting this Fancy Slugcats Mod in the game for funsies. Could someone help give steps on where to go?

[calm pond]

Fancy Slugs is a 1.5 mod, it will not work in 1.9, you'd need to downgrade your game in order to use it