Tell me like I'm 5

I’m a gameplay programmer with a little UI experience but purely on the dev side of things. I’ve got an inexperienced UI artist and I need to help him get up to speed with how to do things with ue4 like best practices, atlasing, how to work with brushes and styles in slate, particularly for use with UMG, and how to do 9-patches or 9-boxes or .9 or whatever the UE4 jargon is for that.