I like this one a lot. I can tell you how I do texture if it helps at all. First it may help to do some studies. Kind of boring to do but go draw a closeup of some tree bark. It's good texture practice. Everything about drawing is observation. Don't draw what you think something should look like but draw it the way it actually looks. For example if you look at a wall you think it's just a solid color but it has little bumps that the light bounces off of making different light and dark areas. Of course it would be a pain to try to draw every single one. A lot of times if I don't want something to look too smooth I use the third brush down on a very light setting and criss-cross it or tap it to create instant texture without a lot of work. Not sure what else to say but hope this might be what you are looking for.