Well I bought organic grounded coffee from my local health food store. Don't think instant coffee is a good thing to use! It was Mild Roast Arabian coffee or something. Smells sensational! Can't remember the brand name sorry. Just try and buy organic if possible.
This link tells you the procedure to doing the enema, and what ingredients, amounts etc you need. I'm in Australia, so I had to convert the amounts for me to understand. 1 quart = 2 cups.
Similarly, this link has slightly different variations of the coffee enema, but they all basically say the same thing.