This was a tutorial I made for someone who wanted to learn how to do wrinkles well. It's nothing much. I apologize if you can't read my handwriting. Yeah. Handwriting is definitely not my strong suit...
By popular demand (and because my handwriting sucks (x_x｡)) I've decided to type what's written on the page. Enjoy!
1. All going downward due to gravity.
2. A good way to add wrinkles is to make a wave and draw an upward line at the low point. Breasts protrude from the body and the shirt should be wrinkled to indicate that. Also, if a shirt covers a girl's figure, make a wrinkle that covers approx. her curve.
3. Another wrinkle due to wave. If the wave is too big, you may need to draw cloth on the other side, so it doesn't look like the clothing only has a front. The crotch and outer leg are shown with wrinkles. If you don't do this, it could look weird. Then again, it might not, depending on the dress.
4. Unless the character is sitting, or in some weird position (I won't ask) pleated skirts look awkward with a lot of wrinkles because there are already pleats.
5. Bunched sleeves will have many wrinkles coming from the right and left, but not really the top or the bottom. The more bunched, the more wrinkles.
6. Loose clothing will have many -long- wrinkles. In a long-sleeved shirt, be sure to make a few extra wrinkles around the joints.
7. Tighter clothing will have tighter wrinkles. I'm not very good at conveyinig this, but just know this tip.
8. Clothing that fits well shouldn't have very many wrinkles.
9. Flexed joints. In this case, the outside will be smooth and the inside will be wrinkled.
(The last box) Jeans. Lines will be thicker and more prominant, but not as many of them. This goes for all heavy cloths. Note the wrinkes at the joint. Also, boys like baggy pants, so draw more wrinkles to show this. Jeans will almost alway sbunch at the bottom, so be sure to make more lines at the bottom of the pants.
Also, draw winkles around a guy's crotch. Their boy-parts make the pants do this.
Girls tend to like tighter jeans (but if your female character doesn't, just follow the rules for the baggy jeans.) They're more form-fitting, so you don't need as many wrinkles. Also, girls are more likely to buy jeans that are fitting to their height, so less bunching at the bottom.