I know this is a late response, but someone else might run into this question. The developer addressed how to install mods if you acquired the game outside of Steam.

https://itch.io/t/836592/modding-and-the-itch-version

They've made it really easy to activate mods by providing an option in the main menu. The game should recognize any mod folder placed in the mod folder in the game directory. In the mod menu you then just need to select the mod you want to activate.