First, I prepared my fabric by placing it in the sink with 1/4 -1/2 cup of Vinegar (depending on how much fabric there was in each wash) & warm water in the sink for about half an hour. Soaking the fabric allows the fibers to open up & Vinegar helps break down the chemicals on the fabric to allow it to dye better. The fabric being wet also allows the fabric to be dyed as evenly as possible.
To get the best possible preparation for your fabric, I have read that Soda Ash works best but I didn't have that on hand.
**For all of these make sure you have newspaper/disposable, or a washable work surface to be able to clean up after. Kool Aid will stain wood, countertops, etc.
Please note that in my experience, red is the strongest color, so if you are wanting a color other than a warm shade keep that in mind!