Sorry for taking so long to reply, but I wanted to do a mini-tutorial to help explain what I do visually. It’s a pretty simple process. Unless there is a very specific pattern I want to do, I usually freehand it straight with watercolor. A pencil sketch would likely contribute to muddying the colors unless done very lightly and without too much detail.
So, first I do a wash and let it dry - that way the following design will be crisp (fig.1). Then I pick a darker color (or a different color, depending - most watercolors are not opaque, though, so you have to consider it when painting) and paint the design with my brush (fig.2). Sometimes I just leave it at that, for a simple brocade/embroidery pattern effect. Or I enhance it with highlights and lowlights - in the particular case an even darker blue (fig.3) and white (fig.4).
The trick to keeping the pattern crisp is to let the layers of paint dry throughout the process so the colors don’t bleed into each other and keeping your hand steady. Hope this helps!