The most simple bentonite clay poultice can be made with just simply mixing in some filtered water to make a thick paste. You can also add an equal amount of Slippery Elm Bark powder to the bentonite clay mixture to soothe irritated tissues and encourage healing in wounds. Other powdered herbs can also be added, depending on the need. The 'glue-like' action of Slippery Elm is especially helpful if you are using multiple herbs in your poultice; it will help to hold all of the other ingredients together.
Here's instructions for how to make an activated charcoal poulitce; you could use this same method with bentonite clay. I don't know why they recommend that you boil the mixture in these instructions...I just simply add enough water until I have the consistency that I desire.