Making a skin overlay with weight/muscle slider support?

I generally understand that you make textures for the low, middle, and high end of each slider (so a fat texture, a thin one, etc), my question is more on how you do it in the sims 4 studio. I don't want to have to make them as swatch options since I want them to be responsive to the sliders. I've seen other skin overlays manage to have slider support and I've even opened them in s4s to see how they did it, but I'm not sure how to do it myself and it's a bit confusing. Is it something I have to edit through the warehouse tab or something?