Here’s a step-by-step tutorial from Tope Abayomi explaining how to create custom keyboards using the iOS 8 custom keyboard extension.

The tutorial explains how to set up your project, using the new keyboard extension, creating a custom keyboard layout, and explains a few of the gotchas along the way.

Here’s an image showing the custom keyboard that you’ll create in the tutorial:

You can find the tutorial over on the App Design Vault.

A nice guide on using the new iOS 8 custom keyboard extension.