other ways are:
Use the double blade knife to cut your stencils. However, you probably can't use it with sandpaper as it would dull the blades. You can make a temporary stencil out of freezer paper or.......
the flexible plastic used for cutting stencils.
Markers are: Multi-Pastel cha;k pencils in either white or light gray.
Or.....can use the spray chalk but then would not be able to remove it from the sandpaper or plastic.
Or if your design is not compicated can do a template out of plastic and mark around it. I find the double blade knif and flexible plastic (name DBK) to be the best if I have a lot to mark.
Also the new ceramic mechanical marking pencils is very good for dark fabrics.