These pieces are primarily two-dimensional and are free-motion machine-drawn. I use the sewing machine with the pressure foot only half-way down and the feed dogs all the way down. This enables me to freely move the fabric underneath the needle. Think of it as if you are drawing with a pen but instead of  moving your pen, you move your paper under the paper. I generally do this spontaneously without a pre-drawn image on the fabric, or I look at a photograph for perspective..