I did all my cars' side windows... Not hard but need experience

The most difficult one is the back window, because it is curved. I did a hatchback myself, the rest - one I drove without tinting on back until sold, all others were left to pros on back ($70).

You can see the color difference on windows from those two shots:
Before
After

In CA, law requires front two windows not lower than 30%, all other windows no limitations. Since the law for window tinting is secondary, not enforced (in every state), virtually you can lower it to what you want. I have seen some shops did whatever you asked them to do (I am not here encouraging you to go beyond law limits). I prefer Limo Dark 20% all around.