You 're technically supposed to make serial dilutions to determine the exact amount. But 1 Teaspoon dissolved in one cup of boiling water will make a good start for 5 gal.
You don't need much, 3 or 4 grams will do it. Break off a piece, soak it in 125ml of cold water for 10 minutes. Heat to 80C/175F, stir until clear and uniform. Pour the solution into cold beer and let it sit for 2 or 3 days. Rack and package your now cleared beer.