How do I get more stars on my island?

Make sure you don't leave any items on your floor (e.g. any DYI), I didn't realise that it had an impact and it was preventing me from getting 5 starts for a looooong time!

YouTube tutorials are very useful. Be sure to talk to Isabelle in town hall for updates too. She'll drop hints. If you're not that far into the game yet, still new, keep doing "What Should I Do" quests for Tom Nook.