F-Droid requires an unmetered connection to the internet. Once you are connected, open F-Droid and swipe down from the center of the screen to allow F-Droid to update it's repositories. This will take a few moments, depending on connection speed.
Next, tap the green magnifying glass icon to open the search page and search for your App of choice. In this example, we will search for AndOTP, a one-time password App compatible with most 2 factor authentication systems.
Tap the blue arrow to download the App, and when the arrow changes, tap the Install button.
You will need to allow F-Droid to install unknown Apps when prompted, tap the Settings option.
Toggle Allow from this source and then tap the back arrow.
F-Droid will now ask permission to install your App, tap Install. Your new App, already fully downloaded should install quickly.
You can now open your new App from F-Droid, the App Drawer or add the icon to your homescreen.