Here's a lovely little Rembrandt drawing! Look at how beautifully this woman's clothing fits her body. Check the drapery of the skirt falling so convincingly from her waist. See how her bonnet really fits on her head!
Now, let me revise those words just a bit. -- Look how Rembrandt shows you the body's form so beautifully. - how he renders so convincing the drapery of her skirt, - how that bit of shadow he added on her bonnet shows us the cylindrical form, even hinting at the bulk of hair beneath. I'm just pointing out the fact that there's a person, an artist doing the "showing"! A person who at some point in his life couldn't draw that well!
I'm stating the obvious when I say Rembrandt was an absolute master! When not working on commissioned paintings he spent hours drawing the people around him or just as likely sat alone drawing his own face by candlelight. He drew constantly!
Here is a section of a drawing where I worked to reproduce the folds and creases of the model's shirt. Aside from the fact that I am no Rembrandt, one reason those folds don't have the strong three dimensional qualities his do is the lack of light and shade, his trademark in his drawings and paintings. There is some sense of form in mine because those carefully recorded lines describe over-lapping shapes but compared with his, these drawings still seem a bit flat. (I almost said, "fall flat" but I don't want you to think I reject these drawings. I like working with line. I value that simple aesthetic.)
You can see another thing going on when you compare with the grey cloth drawing below. I copied as closely as possible the edges and folds of the shirt's fabric and while I did a somewhat similar thing with the lines of the grey cloth, I added both lights and darks on the mid-toned paper. This expanded rendition gives us a lot more visual information so we comprehend the form much more easily.
I've zeroed in on folds and creases here because it's one problem we often face as we draw people. Yes, if you look at clothing carefully you can interpret wrinkles fairly well but as with human anatomy, it helps to understand the anatomy of draped fabrics. Toss a piece of clothing over a sharpe edged object and look at the way it settles itself. Drape it in various ways. Do it again and notice the similarities in the folds from one "toss" to the next. Try different fabrics, heavy and light, then make comparisons. Do some sketches. Do some more!
Another quick exercise is to very lightly crumple a piece of paper and draw it in different views. Crush a few more sheets: one lightly wrinkled, another more heavily. Crush one then unfold and gently flatten it. Draw these using your favorite medium but don't be afraid to try different ones like hard & soft pencils. conte crayon, charcoal, - even pen plus ink wash as our good mentor, Rembrandt, did above but really, -- Just draw!
"There is only one proof of ability - action!" Marie von Ebner-Eschenbach