You know what i find unbelievable hard? wrinkles.

Not in faces, but on clothing. You know, like the folds and stretches in a shirt or pants. I can do it somewhat when im using a model, but sometimes im just practicing drawing people in general without a model and i have no idea where the fold in the clothing go or look like. If you could have a lesson on generally where and how to put folds in clothing, that would be really helpful. Thanks.