I don't like using black-backed 'transfer paper' for replicating sketches: the marks it leaves are too dark and too definite, as they can't be erased. But if you copy a sketch round with graphite on tracing paper, and turn the paper over to rub the graphite onto a new sheet, you've now got a mirror image. Orientation matters because we are used to 'reading' not only words but also images, I would assert, from left to right (even if we do it in an instant). So I might want to grab your eye with a vase of flowers and then quickly lead your eye via the glass bowl with lemons to the apple -- a subtle left-to-right diagonal. Given that I can't use a lightbox even if that would work, since my paper has been wetted and allowed to dry while taped down to a board*, what's the best way to transfer my existing sketch?
What I do here is the following. Starting with a high-quality thick tracing 'vellum', I make my initial sketch or tracing with a somewhat soft pencil (2B in this case). Then I turn the sheet over and place a second sheet of vellum underneath it, and trace the back of the first tracing, so that the graphite will be transferred to the second sheet (and the image is now drawn on both sides of the first sheet). This gives a crisp third image, without scuffing or marring the initial image. I retain the initial image as a more detailed reference, since not every little mark or notation is transferred. Then I turn the second sheet over onto the paper I'll be painting on. As I don't want to etch the paper I'll be painting on, when it comes time to transfer the third image (on the second tracing sheet), I don't use a hard stylus or pencil but instead rub along the outline with a blending stump. If the graphite on the new picture surface needs lightening in places (as in the picture, where it's often a bit too dark), it's easily lifted by dabbing with a small piece of kneaded eraser. Meanwhile, the second tracing paper remains clean and holds sufficient graphite still to be used again for variant paintings.
*If I think I might want to use water, even in conjunction with pastels or gouache, I prefer to be on the safe side and get that paper pre-stretched. Then I can add all the water I want, as the picture evolves, without a concern for cockling.