Draw your image on paper. Then tape lightweight drycleaner plastic over your drawing and trace your image with a Sharpie. Now you can take the thin, clear plastic sheet and position it anywhere you want on your piece to see how it will look. When you like the placement, just trace over the lines with a stylus or dull pencil to transfer the lines to your pot.
If you find your image is too small or too large, just take your original drawing to a copy machine and re-size the image. Then use the plastic bag trick.