One of the neatest features of iOS 8 is the ability to install third-party keyboards which can be used system-wide in all apps. There have been sort of clumsy app by app methods of doing this before but now these keyboards can be your default keyboard.
Installation is simple and it is easy to change once you have installed. The first step is to go over to the App store and find the keyboard(s) you like and download those apps. I have installed Fleksy ($0.99) but others include TouchPal (free), Swype ($0.99), TextExpander ($4.99), and Swiftkey (free). I know there will be dozens of others and Apple has improved their keyboard with iOS 8 but these keyboards may serve your needs.
To install a keyboard you go to Settings >General >Keyboard >Add New Keyboard. You will see any third-party keyboards you have downloaded in the list. Simply select the one that you want to use and Apple will warn you of the dangers of granting developers access to the data that you are typing (i.e. credit card numbers, passwords, etc.) This is basically a warning that Apple is no longer responsible for that information should you choose to use the keyboard. You might want to investigate the specific developer’s privacy policies.
Once you select the keyboard you are presented a box to check with says “Allow Full Access”. This is the box that allows the keyboard to be used with any app. Once you click that, Apple warns you again, saying: “Full access allows the developer of this keyboard to transmit anything you type, including things you have previously typed with this keyboard. This could include sensitive information such as your credit card number or street address.”
Scared? Well, definitely check out the privacy policies of the makers of the keyboard app. Fleksy, as an example, states that Fleksy is designed NOT to receive or log such sensitive data and goes into some detail as to what they can collect and how that data is always separate from a link to your personal data. I do encourage you to read the privacy policies of the keyboard developers.
Once you have given the keyboard permission to have full access you can access the keyboard by simply pressing and holding the “globe” icon on the Apple keyboard and choosing the keyboard. Then you can use those apps to customize your new keyboard. You can switch back to the Apple keyboard either from the keyboards settings panel or by pressing and holding the globe or in the case of Fleksy the “123” button.