I thought for a long time about how to create dark outlines in fabric, that had the qualities of a hand-drawn line. What I mean is a natural line with slight wavers and variations in thickness. You can stitch a satin stitch around the shapes, but a satin stitch is quite uniform in width and has a mechanical quality that wasn't what I wanted. I came up with the idea of cutting the shapes from fabric and fusing them to a dark background fabric. Sometimes that is a black fabric, sometimes another dark color. I like dark blue which has a less harsh look than black, and that is what I am using for the piece I am currently working on.
For fusing I use a liquid called Liquid Thread.
Then I flip the piece over and run a bead of the fusible around the outside edge of the zigzag stitching.
Then I lay the piece, fusing side down, on a teflon ironing sheet and press until the fusing liquid melts and adheres to the ironing sheet. I let it cool for a few minutes then carefully peel it off the ironing sheet. The fusible liquid has melted onto the fabric, sealing the threads of the fabric and making a shiny line.
Now I have a shape to add to my composition, with its dark blue outline and fusible along the edges on the back side, which can be activated with an iron when I am ready to attach it to the rest of the composition.