After the background has been marked with a soft pencil, I stitch a basting stitch along the pencil lines, using a consistent dark purple thread. When I see the purple thread, I recognize is as the basting thread.
UPDATE: Do not use a dark thread. It can leave a residue. I now use a lighter colored thread for this.
When it is time to remove the basting thread, I know which thread I have used for this purpose. The basting stitch outlines the area to be appliqued, creating a very accurate applique design on the right side of the fabric (which is the side displayed here). I can let this design sit for a long time, if necessary, before working on it. The pencil marks wear off, and the basting remains. The basting can easily be accomplished in a short time and doesn't require any special concentration efforts. If you want to correct the design, you have 3 opportunities: 1) on the pattern copy, 2) on the back of the block, and 3) on the front of the block when you do the applique.