The app will already give you instructions for taking the pictures, but we repeat some hints here so that you are prepared. To make your pictures comparable, you should always:
wear only shorts (and a sports bra for women)
take them ideally in artificial light, in the exact same spot
remove any distractions in the background, for example, stand in front of a white wall
Stand upright, with your feet shoulder-width apart. Look straight ahead. Arms hanging by your sides.
Align your knees and eyes with the respective guides