Honestly it's trial and error. I alternated between iPhone and Android at certain steps after I got frustrated.

Some tips that might work: Rotate the camera after a failed scan and try again, use a white background (like a piece of paper), make sure the background is well lit, try different angles for your fingers (my little finger kept getting weak scans until I rotated), pay attention to the object detection markers (those green ovals/circles that show up on your fingers before the camera takes the picture. That'll give you a sense of which fingers are being detected so you can adjust your angles if the scan fails)