Add 1/2 cup of lotion to a bowl. Add 1-2 drops of the food coloring (if using concentrated gel, if using regular food coloring you'll need more drops to color) and mix.
Mix the lotion and food coloring until combined.
Add 1 cup of cornstarch and mix.
Mix until combined.
Then take out the dough and place on some wax paper. Knead with your hands (you may want gloves here so your hands don't stain). Keep kneading until it forms a dough-like consistency (this may take 5-10 minutes). It will feel really sticky at first, just keep kneading. If after kneading for the full length of time it is still too sticky, add another tablespoon of cornstarch. If it's too crumbly, add a tiny amount of lotion in at a time and knead it in until it becomes the right consistency.
Video
Notes
If your dough is too crumbly or sticky: If the dough is too crumbly, you need to add more lotion. If you find it gets too sticky when you add more lotion then you are adding too much lotion. Add a little bit at a time and knead it in. You will need to knead it for quite some time in order to get it to the right consistency. If it's still too crumbly, then add a tiny bit more lotion. If it's still too sticky, then add a tiny bit more cornstarch. Repeat the recipe for each color you would like to make.