When you install Cuneiform it will be added to your list of available THIRD-PARTY KEYBOARDS.
Go to Settings, General, Keyboard, Keyboards, Add New Keyboard ...
and tap Cuneiform in the list of THIRD-PARTY KEYBOARDS.
You can switch at any time between Cuneiform and any other available keyboards, by tapping the globe 🌐 key on the current keyboard.
The Cuneiform app is also installed which allows you to change settings and customise accents and suggestions.