Like putting make-up on and dressing up.
Or when you customize something to make it look pretty.
Or cleaning something up.
It's the act of making something look better.
Alice " I'm sorry, I needed to prettify myself some more."
"I prettified my room, I was tired of the old look"
"I had to prettify my Xbox before listing it on Ebay"
"My girlfriend is coming over, hurry up and prettify the apartment"