In order to activate a farm, you must first find the raid portal in your town. Once found, pick up an offering and stand close by to it for 10-12 seconds before walking away. After that is done, go into Beat Sabers main menu and choose Farm. You will now see 2 new options on screen - 1) Farm Raids & 2) Equip Item which you would use if you already have items tied up with farming or want to re-assign them somewhere else.