Hi there don’t worry. It is super easy.
Just click on the following link: https://testflight.apple.com/join/71CdOvmG

1. Install “Testflight” because we are still in beta.
2. Then go back to first page.
3. Click on the button as display.
4. Allow/ Accept the Testflight stuff, if you are okay with them
5. Install marble