Short answer: install the keyboard's app, then go to Settings → General → Keyboard → Keyboards → Add New Keyboard and pick it. Tap it in the list and turn on Allow Full Access. In any app, long-press the globe key and choose the new keyboard. The whole thing takes about a minute.

What an AI keyboard is

An AI keyboard is a third-party iOS keyboard with tools above the keys. Instead of copying a message into a chatbot and pasting the answer back, you select the text in the app you are already using, tap a tool such as Proofread or Reply, and the result is written into the text field. It works in iMessage, WhatsApp, Instagram, Gmail, Slack and dating apps, because iOS lets you use a third-party keyboard almost everywhere.

Step by step

Install the app that contains the keyboard and open it once. iOS only lists a keyboard in Settings after its app has been installed.
Open Settings → General → Keyboard → Keyboards.
Tap Add New Keyboard and choose it under Third-Party Keyboards.
Tap the keyboard in your Keyboards list and turn on Allow Full Access.
Switch to it. In any text field, long-press the globe key (🌐) at the bottom left and select it. Tapping the globe once cycles to the next keyboard.

Why does it need Full Access?

iOS blocks third-party keyboards from using the network unless you turn on Allow Full Access. An AI keyboard has to send your text to a model to rewrite it, so without Full Access the keys work but the tools cannot. iOS shows the same generic warning for every keyboard, so it is worth reading the app's privacy policy to see what it actually does with the text you send.

Where it will not appear

iOS always switches to the system keyboard in password fields, and some banking and phone number fields do the same. A few apps block third-party keyboards entirely. That is an iOS rule, not a setting you can change.

Getting good results from the keyboard

Select the right text first. For a reply, select or paste the message you received, not your own draft.
Add one line of intent. "Say yes but not this week" is the difference between a usable reply and a generic one.
Use the narrowest tool. Proofread fixes mistakes without changing your voice. Change tone keeps what you said and changes how it lands. Paraphrase keeps the meaning and swaps the words.

Troubleshooting

The keyboard is missing from Add New Keyboard. Open its app once, then check again. Restarting the phone fixes the rest.

The tools spin or do nothing. Full Access is almost always off. Settings → General → Keyboard → Keyboards → the keyboard → Allow Full Access.

It keeps switching back. iOS remembers the last keyboard per app. Switch once in each app where you want it.

Using TextAI as your AI keyboard

TextAI's keyboard puts 28 tools above the keys, including Proofread, Reply, Change tone, Shorten, Summarize, Instagram caption and Icebreaker questions. The setup guide has the same steps with screenshots of where each switch is, and the full tool list shows which tools are on the keyboard.