Bentonite is a negative charged, clay-like mineral that removes positively charged particles. It works best when the wine is at warmer temperature so it is suggested clearing with bentonite in the late spring or early summer. It is also more effective at a lower pH because the positive charge on proteins is stronger at lower pH levels. It is usually added prior to fermentation, sometimes during secondary.
When making a wine kit from concentrate you will often add the bentonite on the first day. This happens for a couple of reasons. It is used as a nucleation sites for the removal of CO2. Additionally, the bentonite is moved all around the wine by the CO2 making it more effective in a shorter time period.
Generic low level fining for five gallons: blend 1/2 tsp (approximately 3g) with 1/2 cup of hot water (140 to 200 F) in a blender for 1-2 minutes. Let stand for 60 minutes, mix, and stir into wine. Let stand for 10-14 days and then rack off. This is fining at .16g/L
For a generic mid level addition: Follow above directions but use 9 grams of with 3/4 cup of water. This is fining at .5g/L.