A tracing wheel is used to transfer patterns from pattern paper to fabric when sewing. Why not also use it to draw geometric lines and patterns for sashiko-inspired mending? :)
Use in combination with carbon paper/tracing paper, which you can buy here. It comes in 4 colors, so you can find a color that shows up on your project!
Place the paper with the colored side down against the fabric you are marking (traditionally, sashiko is done from the inside; in these pictures, I demonstrate on the outside). Use a ruler and roll the tracing wheel over the carbon paper. You can use graph paper or draw your own patterns to follow. Then you can place the patch on the inside of the damage and sew it in place with running stitches that follow the markings from the tracing wheel.
The marks should wash off, but it's a good idea to test on a less visible area first, or only mark on the inside of the garment, or on a patch you attach to the inside, so that the marks won't show anyway, only the stitches that go through the patch and garment.
It may be beneficial to use long sashiko needles and a thimble in your palm for this type of repair.
Do you want a tracing wheel with a serrated edge? You can find it here.