What RPM should my car idle while driving and what RPM should I shift?

I have a 2003 Ford Mustang V6. 5 speed manual transmission. This is not the first manual car I've owned, but I'm curious. I never really look but I've read in some places that in order to save gas you should shift at 2 RPM before I used to shift at around 3-3.5 RPM. Also what's a good RPM my car should idle at while I'm in gear? Let's say I'm driving in 3rd gear. What's the ideal RPM to stay at before needing to shift.

There's an emissions sticker under the hood. It tells you the exact idyl speed for your car