Once purchased you’ll be given a link to download the TestFlight version of the app. Open that up on your iPhone and install the TestFlight app if you haven’t already. And then once you’ve got that installed you install the app from TestFlight via the link provided when you purchase the app.
When you first install and run the app, you’ll first need to login with your account details. Once you’ve logged in you’ll need to complete some initial configuration. First, navigate to the app’s settings, then access the general settings menu, and set the theme to Theme 3.