This app is designed to give you information on all of the Archangels, their purpose and how each one helps us, so you will know who to call on when you are in need. There's also information on which crystal to wear or carry with you to help you attune to their energies.
Remember that you can never call on them too often, or "overload" them as they are able to help multitudes of people simultaneously - they are there waiting to help you, and love to be asked.
Simply call on them by name, either out loud, or just talk to them from your heart.