How do I paint a metal outside door so it doesn't rust or peel?

https://www.youtube.com/watch?v=CGE_GtD0koU

Take the door down, sand lightly , use a primer and then an outdoor paint in your choice of color. I hear that semi gloss or gloss lasts longer on exterior doors.

sand, clean, sand again, Vaccum and paint in the shade in DRY weather

As others have said, it is all about the prep work. Pay special attention to all corners and cracks to make sure they are meticulously clean. I'd also use spray rather than a brush because I believe it is easier to create a continuous film with spray paint.

Use a good metal primer like Rustoleum. If you can't take the door down, be careful with drips.

I spray-painted my doors after sanding and priming. (I couldn't remove them by myself, do I chose a nice day, and used drop cloths to contain my painting.) I used Rustoleum spray paint and used several light coats. They work far better than heavy coats of paint. Gloss or semigloss are more durable.

you need to purchase a paint that is specifically formulated for metal doors...and paint when it isn't too hot