After clicking the button of the app you would like to install, confirm it's installation when prompted.

You can see the progress of the installation process by returning to the home screen.

Once installed, the app's name and icon will now appear.

You won't be able to launch the application until you trust the developer.

Go into your iPhone's Settings, then navigate to General.

Select Device Management.

Select Golden State Overnight Delivery Service.

Then select "Trust Golden State Overnight Delivery Service".

Confirm the trust.

You should now be able to launch the application!