Measure around the thickest part of your thigh. Divide that number by 2, and cut out a piece of cardboard with that measurement as the width. The length of the cardboard depends on how long you want the melted "slime" to be.
Cover the cardboard with plastic wrap.
Put the cardboard inside your tights carefully. You will be painting on top of it.
Using the fabric paint, start out by drawing the outline of the melting.
Fill in all the empty spots.
To make the paint look neater, I cut up an old credit card and used it to smooth out the paint, kind of like frosting a cake. After you are done with one side, let dry completely (4+ hours) before flipping it over to the other side.
Repeat with the other leg. After all of the paint dries, carefully take the cardboard out. Some of the plastic wrap may stick onto the tights, but you can peel it off. I would recommend waiting 72 hours before washing the tights.