As I see it, the two main approaches you can take for doing wrinkles are (1) putting a thin skin of whatever modeling compound you're using onto the figure and then drawing or pinching the wrinkle shapes with tools or (2) making a bunch of thin "snake" shapes out of the modeling compound, laying them on the figure wherever you want the wrinkles, and then smoothing/blending them in with tools or your fingers. Or you can combine both techniques.
I've never tried it, but I suppose you could make a wrinkle texture stamp and use that to rapidly imprint a wrinkled texture onto the surface of your modeling compound.
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I've never tried it, but I suppose you could make a wrinkle texture stamp and use that to rapidly imprint a wrinkled texture onto the surface of your modeling compound.