The picture above shows the cable as a motif with rounded ends rather than being connected to another part of the quilt design. I'm showing it on paper so I can show the steps for marking it. The broken lines are part of the marking process and aren't quilted.
To get started, you need to look at the space that will be filled with the motif. This diagram shows the space created when the larger parts of the quilt design come together. There are lots of patterns that would work here but the cable as a motif is kind of unique, even for Welsh quilts.
The next step is to figure out how big the motif or curved cable will be so I usually mark the general shape right on the fabric of the quilt top (as shown with the sausage shape marked with the broken line in this next picture). On the red sateen quilt, I've been using soap slivers to mark which stay long enough to quilt and can be marked again if needed. Soap makes a great marking tool but be sure to use soap that IS NOT a moisturizing soap which will probably have oil in it.
So now its time to mark the cable itself.
I've marked a broken line down the center of the shape and I've decided I only have room for three sections on this cable so I've also drawn 'X's on the line where the cable lines will cross the center line to give me three sections. Now the cable is drawn using the 'X's as the guide. You can see in the next picture that the lines of the cable have crossed on the 'X' and the ends of the cable are rounded.
This is all there is to marking a really simple motif pattern. They can be as long as needed or just a short one like this one but remember that it's not about drawing and quilting the pattern perfectly, each section will be a little different from the others but that's what makes them interesting in the finished quilts. Very useful little pattern. Enjoy.