Whole document in the frame.
Make sure to fit the whole document in the frame to ensure accurate cropping. Only one document at a time per each frame.

Some contrast needed.
Some contrast between the document and the background is needed so that Snapter can detect the page and know where to crop. Any color background will work (as long as it is rather uniform and does not have much color pattern), but black or wood normally work best.

Hold camera steady.
Hold camera steady during snapping a picture to get a clear result. Tripods or an anti-shake mode would help considerably. Using Macro mode also would improve the focus of the picture..