As Dekka and others said, the answer is to look very carefully at how light and shadow fall on RL fabrics. Don't think of it as "how do I paint a wrinkle". Think instead about how the lighting and shading looks as the fabric bends and folds in space. As the best art professor I ever had used to say all the time, always approach every drawing/painting simply as a problem of light and shadow, and you'll never go wrong.
The best advice I can give you is to do drapery studies. For centuries now, artists who wish to draw and paint realistic clothing have spent entire lifetimes doing exactly that. Learning to paint convincing fabric is a skill that takes patience and practice to develop, and ongoing practice to maintain.
Here's a drapery study Da Vinci did when he was around 20:

And here's another from when he was closer to 30:

It was those studies that enabled him to paint paintings like these:


Doing similar studies yourself will teach you how to paint convincing wrinkled clothing for SL or any other environment, like these pants, for example:

Notice how the light and shadow fall on the folds of the pants. It's exactly the same as how it falls on those draperies.
Grab a bed sheet, throw it randomly over a chair, and draw it. If you've never done this kind of thing before, be prepared for your first few attempts to be pretty bad, no matter what your current level of artistic skill. With repeated practice, you'll get better and better at it. Absolutely no one (not even a master like Da Vinci) has ever been good at it right from the start. It's a skill, not a talent, and as such, has to be developed.
If you're really serious about this, I'd highly recommend you enroll in a drawing/painting class at a local college or adult learning center. Make sure the instructor knows you're especially interested in drapery studies. Trust me; it'll be well worth your time.
All that said, don't get me wrong. I'm not trying to say you can't get decent results after reading a tutorial or two. You certainly can, if you've already got artistic skill and experience. But I take it from the way you worded your question that you're probably lacking in experience. The only way to change that, obviously, is practice, practice, practice. And drapery studies are the absolute best way to get it.