Was patching some holey jeans last night, with ugly hand-stitches and iron-on patches.
Some of the holes were inside the back pockets, so you're supposed to cover the hole with a piece of paper to cover the glue.
It helps if you use a completely blank piece of paper.
Not a mostly-blank piece of paper that was printed off a laser printer.
This is mostly because if you hit the printed part, the iron will transfer the ink off the paper and bake it into the jeans.