Depends on both the desired texture and the style you want to achieve, but in any case, you need to look carefully at exemples first (real-life or photos), to get a solid grasp of how things work. Notably, in case of textured surfaces, you need to learn how the light interacts with the texture, where the shadows fall, how reflective the material is, and so on. Observation is key in drawing.
Then, after you've understood how the texture works enough to be ready to draw it, you have two options:
- If you're using the default round brush, then make this brush very small, lower the opacity (anything between 50 and 70% opacity should be enough), and try to emulate the texture by making tiny dots, scratches, lines, etc. in every areas where the shadows should be, in a darker color than your base color. This gives the most accurate and photorealistic results, but it's time-consuming and some pro artists may look down on such a workflow.
- If you have textured brushes, then simply select one with a similar texture (at least from afar, to give the optical illusion that it's the same material) and make random strokes with it on top of the part of your drawing that needs texturing. Depending on the brush you use, you may have to lower the opacity or mess with the settings until you get a convincing result. It's much faster and easier, and the look of textured brushes seem to appeal to a lot of people nowadays.
...Unless you were talking about patterns (like the ones you'd see on fabrics, since you mentioned models), in which case it's another process.