When you're adding it to the primary, the directions are different than if you're using it post ferment. You need less, when using it in the primary. Here's what we do: use 1 tbs per 1/2 cup hot water--this is enough for 5 gallons. Just mix it up and toss it in to the primary.
If you're using it post ferment, use 2 tbs tp 1 cup warm water--this is also for 5 gallons. Let it sit overnite, then add to carboy. You have to be sure it's fully hydrated.
At the primary, be sure to stir it up every time you're punching down because it won't work just sitting on the bottom. The agitation gets it in contact with the wine, and as it comes up thru the must, the charged particles of the bentonite grab onto the opposite charge of the proteins, debris, etc and bind it all together. When you go to siphon off the primary, you're wine will be as ugly as dishwater and you'll wonder why you ever used the stuff. Just have patience with it--after several weeks, you'll see how well it starts to clear. You'll need a couple more rackings when using bentonite, so just keep your eye on the carboy and rack when you get and inch or so of lees.