1. Subscribe to the Ravenfield mods you want via the Steam workshop
2. Make sure you are on beta - EA26 for the game by going to the game's Steam page in your Library, click the Settings gear, choose Betas, and enable "beta - EA26".
3. Start the game. Go to Play, Instant Action. Then there is a screen to config the match, in the lower right of that screen there is a button called Mutators. Click that, it lets you enable all your mods.
4. Have fun, there are many cool mods!