First thing you need is a cup with some oil. "Some" is the kind of measurement that makes me cringe, but I promise it's not crucial to the activity. A little bit. I filled the glass a little less than halfway. You then drop in a few drops of food coloring -- maybe four or so drops of each color -- and watch as they form little balls on the bottom of the glass.
Next up is the stirring. You have to stir, stir, stir until the little balls of color break apart and become tiny. Ethan loved this part.
Once stirred, you dump the glass of oil into a large glass or vase -- I used an apothecary jar that I use to store bath bombs -- that is filled almost to the top with warm water. Not hot, but warm. A little warmer than room temperature.
And then after a couple second delay, the firework show begins.
If you use less colors (and less drops of said colors), it takes longer for the fireworks to eventually turn the entire jar a dark, solid color. Ours was the perfect amount of time before Ethan lost interest anyway. Toddlers are funny like that.