Use even light and light paper
Place the page flat and avoid a hard shadow from your hand or phone. Warm paper is fine, but strong folds and dark corners can look like ink.
Keep the camera parallel
Shoot straight down so strokes do not become stretched. Fill the frame with one signature and leave a little margin around it.
Crop nearby marks
Remove pens, stamps, printed text, and table lines before upload. The tool cannot know which nearby mark belongs to the signature.