Click on Units. See the tab for Character Upgrade? Click on it. Pick a girl. Press select. Now click on Add materials (any of the 10 of them). Go through until you find the manyu that matches the girl's color. Click on them. You can click on up to 10. You can also click on other girls, bloom dragons, and whatnot. For now, don't click on anything but a girl you're fine losing or a manyu. Then click on Upgrade. Say yes. She'll now get xp based on which girls and manyu she got; higher level girls and manyu supply more xp.

That's it.