The tool "Flat Camera" is intended to use as a scanner. But instead of a separate scanner unit you use the built-in Android™ camera.
You put the painting or document horizontally. Outside, when cloudy, is best because then the light is better.
The tool's arrows will guide you to when tilting the device flat.
Make a sound when you want to take the snapshot. This is an advantage since your fingers might be busy holding the camera.
Images from the app