After the keyboard extension was installed, bring the keyboard up:
Open the texting app (can be any texting app; Optimized for the native Messages app)
In the texting screen, tap the text field to bring up the standard keyboard
Tap & hold to Globe icon that is next to the space bar
Choose Adult Emoji 💋 in the menu that pops up.
(if the keyboard isn't listed here, it's not installed. See "How to Install Keyboard" article)
To Use the Keyboard Extension
Once the Keyboard Extension is up, tap any emoji and Paste it. The galleries are at the bottom.
Gallery sorting, contents and unlock status is imported from the main app.
When You're Done
When you're done and would like to switch back to the standard keyboard, just tap (no need to tap & hold) the globe icon that is next to the space bar.