How Do I Add A Custom Keyboard For My iPhone?
Starting in iOS 8, you can download third party keyboard apps to your iOS device. Once you have downloaded a keyboard app, open the Settings app and navigate to General > Keyboard. Select the keyboard you downloaded then switch "Allow Full Access" to the on position.
To use the new keyboard you added, open any app that allows typing so you can bring up the standard iOS keyboard. Tap the globe icon at the bottom of the keyboard and then choose your custom keyboard.