The knight's clothing is pretty plain (a lot of white) so I wanted to add a simple design along the border of the cape to add a bit more interest. From the standpoint of historical accuracy, it's certainly possible that he could have something like this. I started the design along the bottom a while back but still needed to continue it up the side and a bit on the front. So I thought I'd take this opportunity to show how I approach something like this. The first step is to figure out what sort of design you want to do. I'm continuing what I already had, so no need to worry about that. With a repetitive design you want it to look as even and consistent as possible. This is always tricky when doing it by hand (without a template of some sort). To help I placed a mark where each wave gets closest to the edge using a rule to keep the mark spacing consistent.
Next I start to fill in the lines. Break down the pattern to simpler parts. In this case I began with the portion crossing over the other line. Note that if the line cross through the right dot, it will be slightly offset from the left dot.
Next I put in the other side of the line, following the original one and trying to keep the distance between lines consistent.
Now it was a simple matter of connecting the lines going under to fill in the rest of the pattern. Where necessary I adjusted the original marks so that they looked like they meet. It's a bit rough here and that's okay.
So with the design sketched on, now it was time to go in and clean it up. Taking the background color I carefully went along the sides of the design lines. You don't need to go everywhere, just where the lines are too thick or not smooth enough. I also did some glazes of the cape colors to blend it all together and add a bit more shading/highlighting.
And here's the whole scene