Download and install the keyboard as an app as you do with other iOS 8 keyboards, and then enable it in settings. When you open the app, you'll be prompted to create an account so that you can upload your own GIFs and personalise the keyboard. But you don't have to.
Actually, using the keyboard is easy enough. Choose an emotion, select one of the available GIFs. It will copy to your clipboard, and then you paste it into a text box. It's not the first GIF keyboard. But it's the best. You can get it here.
This is what it's like to use it:
At first, I'm like hi!!
This is exciting!
But then I got to thinking...
And I'm troubled that it's so hard to express the complexities of my emotions and ideas with only a smattering of reaction phrases.
I feel defeated.