Hmmmk, so here is what you need to do, not sure if you are looking for fabric wrinkles or skin wrinkles, so I will explain both.
FABRIC WRINKLES:
-Cover area in base color.
-Shade base color based on the in game shape of the area.
-Take either a black with low opacity, a darker shade of the color you are using with higher opacity, or a small burn tool. I personally use the black with low opacity.
-Make some thin little dark lines running along the fabric in the way it flows.
-Use a lighter brush than your base color and make some little highlights running along above the dark lines in places where the fabric would bunch up and make wrinkles that stick out more.
-Use blur or smudge(I prefer smudge for this application) and feather the highlight into the base color a little bit.
-Use darkish shades to make the darker area bigger under areas where the fabric sticks out and creates shadows.
-Tiny touch ups with smudge tool and possible a little dodging in areas where it doesn't look natural.
SKIN WRINKLES:
-Take a tiny burn tool, size 1-2 and make some little reference lines where you want the wrinkles.(Remember, wrinkles aren't perfectly straight lines, they have little branch-wrinkles and stuff sticking off of them.)
-Do some VERY tiny highlights on tops of dark areas.
-Work the wrinkles into the skin a little with highlights around tiny areas of depth(crows-feet around eyes)to give it a sunken-in feeling.
-Darken areas in which a wrinkle would prevent light from touching.
-Once again final touch-ups with tiny smudge in unnatural looking areas.
-Looking at a picture of an old lady's face would be a good idea so you could see what they look like in real life.