The page edges aren't found, or the page looks crooked

1. Put the page on a plain, darker surface, so its edges stand out.
2. Use even light and avoid your own shadow over the page.
3. Hold the phone straight above the page, not at an angle.
4. After taking the photo, drag the corner handles to the page corners yourself.

A crumpled page scans better if you flatten it under a book first.