One approach I came up with is to doodle in shapes and then apply the shapes to cards. I apologise for the quality of the images. I was simply too lazy to do them again when I saw how dark some of them were.
I tried the technique on some 8 x 10cm cards I'm making for another project (which I'll post a few days from now).
I started out by drawing or tracing shapes -- hearts, circles, and ovals, on watercolour paper scraps I had left over from a different project. Any heavy paper or thin card will do, though.
I then "took a line for a walk" across all the shapes. I found it made nicer patterns if I was able to make wider sweeps. All I did was draw random loops, scrolls, and circles, going over the lines. The scribbles outside the lines of the shapes are simply cut away in a later step. I got the nicest outcome when I drew the lines with a cheap gold gel pen. Euro/dollar store gel pens are plenty good enough for this project.
The next step was to glue the shape to a piece of light card for stability and then to cut out the shape. This removes the "over the edges" lines.
The final step was to attach the shapes to the cards, which I first stamped with one of the scrolls from the Aud Design stamp sheet I mentioned in the post above. It's not easy to see in the photo at the top of the post, but I used sticky foam pads to attach the shapes, so they are raised.