Stole this from How to geek.
To enable a new keyboard after installing it, you’ll need to head into Settings after installing it. Navigate to Settings > General > Keyboards > Keyboards. Tap Add a New Keyboard and select the keyboard you just installed.
You can enable multiple third-party keyboards here so you can easily switch between them later.
Give a Keyboard Internet AccessTo give a keyboard full Internet access — for example, allowing SwiftKey to synchronize your typing profile across all your devices — tap the name of the keyboard you added on the Keyboards screen and enable “Allow Full Access.”
Apple notes that this could be a security issue, as the keyboard could potentially transmit private data you type — such as your password or credit card information. Keyboards don’t have Internet access by default for this reason, and you don’t have to enable Internet access to use any keyboard. Some keyboard features — like SwiftKey’s syncing — may only be available with Internet access.
Switch to a Third-Party KeyboardOnce you’ve enabled a keyboard on the Settings screen, it will be available to use. Bring up the keyboard in any app and tap the globe icon in the bottom-left corner of the keyboard to cycle between available keyboards.
You can also long-press the globe icon to see a list of available keyboards — the same list you configured on the Keyboard Settings screen. Tap the name of a keyboard to switch to it.
Use the keyboard normally and, when you want to switch back, tap the globe icon again. Apple requires that all third-party keyboards include a button that switches to the next keyboard, but it will look a bit different on each keyboard.
Configure a KeyboardEach keyboard will also install its own app icon on your home screen. You can open the app to view settings and configure the keyboard. The apps will often contain information about using the keyboard, too. To uninstall a keyboard, remove the app like you normally would — long-press the app icon on your home screen and tap the X that appears.
That’s basically it — this process will be the same no matter what third-party keyboards you install in the future. The ability to enable multiple third-party keyboards and cycle between them with a single tap is convenient — it allows you to install more specialized keyboards and quickly switch between them without opening any menus.