Use plain, light paper
Place the sheet flat and avoid hard shadows from a hand or phone. Warm paper is workable; folds and dark corners compete with the ink.
Keep the camera parallel
Shoot straight down so loops do not stretch. Frame one signature and leave a small margin around the complete mark.
Remove nearby marks before upload
Crop pens, stamps, printed text, and table lines. The tool cannot know which neighboring mark belongs to the signature.