tinting windows requires a whole new set of skills that are different from using tools and following directions. a lot has to do with experience and knowing the material and have delicate hands.
I have tried to do all of my windows. I'm happy with how my side windows turned out. there a few small bubbles (1mm to 2mm) here and there and they are not noticeable unless you stare at them. they weren't something that were visible until the tint was cured after about a couple of days so i couldn't fix them and it was clear it had to do with my skill
the the back window is a different story. i have ruined 3 sheets of tint and still can't get it perfect. the curvature is greater so you have to dry shrink it more than the side windows. proper anchoring is the key so you don't get excessive corners. i recommendation ordering a few extra pre-cut sheets so that you can keep on doing it until you get it right. because in my case every time i ruin a sheet and i feel like i have learned my mistake, but by the time i order an sheet and it arrives i have forgotten what i thought would've shrank it better.
anyways, in my experience, you won't be saving much for a subpar tint job. so unless you are interested in the experience i would pay an expert.