Process is simple enough. Soapy water, squeegee, sharp blades and a heat gun. There are a ton of videos on YT on how to DIY it.

Spring for the ceramic tint. Its better than regular tint. It blocks IR, which regular tint does not. Really makes a difference if you live in a hot climate.

Depends on the kind of car. If you have a sedan with a really angled rear window, it can be a pain. There's a reason you see so many cars with bubbly tint on the road.

It can be a fun project and great practice on your own car, but I've never been able to do that kind of stuff properly enough to feel comfortable doing it for others.