After downloading the app, you can simply start reading the chapters that are shown, checking off each one as you read them.
When you finish all the items, you can move to the next day.
The app is setup to provide a reading list for the day, but it is up to you to decide how much to read. This is just a suggestion. You can still follow the reading plan even if you choose to read more or less each day. The app just keeps your progress so you can pick up where you left off.
You can even split the chapters throughout the day, so you can read a little here and a little there and work through the chapters at your own pace.