Finings is a clearing agent to help improve the clarity of your beer. You can fine with gelatine in either a fermenter (primary or secondary), or directly in the serving keg. In either case, you want the beer to be cold, ICE cold. The colder the beer is, the more haze-forming particulates will form. This is also known as Chill Haze. The more haze-forming particulate that forms, the more particulate the gelatine can fine out. You may have some success at cellar temps (10-12C), but for best results the beer should be at serving temp (0-4C) or cold crashing for -1C for 3 Days.