To obtain the Expense mobile app, and to set up your account to use it, see Enabling Your Account for Mobile Devices.
To get the best results when capturing digital images of your receipts:
place the receipt on a flat surface, with good colour contrast between the surface and the receipt
keep the receipt as flat as possible, with no (or minimal) creases
zoom in to make the receipt as large as possible in the viewfinder
keep the camera steady when capturing the image - check the result to make sure it’s readable