What are good tips on how to shoot a short film with a smartphone camera?

The main thing is, start with a good simple but solid story, and plan out as much as you possibly can. Do shot lists, number your scenes, draw storyboards… basically, create your movie on paper before you shoot it. A fantastic and FREE tool that helps you do this is Celtx (https://www.celtx.com/index.html). This is what all great filmmakers do, and Hitchcock famously said that shooting the actual film after all the prep is done was the boring part.