Something that you'll often have to do in fashion design is create pattern or texture on the fabric of the garment that you're designing. I'm going to go with polka dots in this dress. So it can go a lot of ways, you can be very graphic about it, or you can be very literal about it. I'm going to go with the graphic approach. I just start out with darker version, I'm going to do darker blue polka dots on a lighter blue dress. You want to just keep in mind like what your spacing is.
Most of the time you'll have a swatch of your fabric right beside your sketch, so people will get it, but it's just all about just creating your texture, or your print, or what have you. A lot of times you don't even have to cover your whole dress, you just do a section of it, that's why it's really important to have your swatch of fabric beside your dress. I don't have a swatch of this fabric, but whatever, I guess I'll do the whole dress then. I'm trying to like keep in mind the folds, going with a graphic quality. As you get to the edges of the dress, you go darker with your polka dots, because that is creating more shadow.
And if you really want to, you can go in afterwards with your pen and outline every single one of these polka dots, but I really don't think it's necessary, I think they look fine as they are. And that's how you draw a pattern or print on a fashion illustration.