Finings is a clearing agent to help improve the clarity of your beer.
You can fine with gelatin 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 gelatin 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.
-Get a microwave-safe glass cup (Pyrex measuring jug). Measure out 150mL cup cold water from the tap. (Potable water)
-Add one teaspoon of gelatin, and stir the solution.
-Place the water/gelatin mixture in the microwave, and begin to heat it 15-30 seconds at a time, stopping to stir the solution and check the temperature. As it heats up, you'll notice the gelatin will begin to dissolve.
-Continue heating/stirring until the mixture reaches somewhere between 65 - 75C remove from the microwave. Don't allow to go above 75C or you'll be making jelly. This is just pasteurisation (killing the germs)
-Give the mixture one last stir, and dump it straight into your beer. Gently swirl the fermenter or keg, and return it to your fridge or kegerator for 24-48 hours.
-If you used a keg, purge the headspace with CO2 to remove any oxygen that got mixed in.
Gelatin works rather quickly. It's extremely effective at dropping yeast out of solution, as well as lots of haze-forming particulates. I find that 48 hours later, the beer drops crystal clear. If you bottle your beer, rack the beer to a bottling bucket, and bottle away. Don't worry, there are still plenty of yeast in solution to carbonate the bottles.
If you keg, you'll have to draw off a couple very cloudy pints before the beer clears up. Gelatin literally drops yeast and particulates down to the bottom of the keg. Since the dip-tube draws from the bottom, that junk will be the first thing pulled from the keg. After a pint or two, it'll be smooth sailing until the keg kicks.