Setup a Key Mapper keyboard
On Android the on-screen keyboard that you see when you want to type something is called an 'input method'. Key Mapper uses its own input methods so that it can type stuff or press buttons for you in other apps. You can not use the input method that you are usually using (E.g Gboard or Samsung Keyboard) when you are using some features in Key Mapper (e.g key code actions) since Android only allows one input method to be used at a time.
'A Key Mapper keyboard' through out this documentation and the app refers to any input method that works with Key Mapper. There are only 2 right now.
The one built-in to Key Mapper is called the "Key Mapper Basic Input Method". This one has NO on-screen keyboard. You won't see a keyboard when you want to type something. This keyboard doesn't support non-English languages! You are not recommended to use this since it is not a proper keyboard.
If you want an on-screen keyboard to be available while using Key Mapper or if you need support for non-English languages, you need to install "Key Mapper GUI Keyboard". It is available on the Google Play Store, F-Droid and GitHub. This is the recommended input method for Key Mapper.
'Enable a Key Mapper keyboard' means turning on the input method in your device's 'Language & Input' settings. The location of these settings is often called something else.
'Use/choose a Key Mapper keyboard' means making the Key Mapper keyboard that you have installed the one that shows up when you want to type something. Multiple ways of doing this are described here. Android allows multiple input methods to be enabled but only one to be used at a time.