I don't think you are suppose to use instant coffee at all. Wilson's coffee is suppose to be the best because it is specially made for coffee enema purposes. But some people just use organic coffee. Make sure it's not decaffeinated, and the organic is important. Non-organic coffee has a lot of toxins in it.
Here's a link on how to do it, although with the wilson's coffee, they give a slightly different procedure, for example, you simmer the coffee a little longer. Also, with the wilson's coffee, you can get a five dollar enema bucket which is easier to use than the regular enema bags. It can be used for a long time as long as it's cleaned good regularly.