Well the only tips I can give you are:flawless and practice
Let the firework turn into a star and hit it at the last minute before it disappears
and it will be flawless. If you keep hitting flawless then you flawless bonus
will increase and you will be getting 24 points each firework you hit. If you
mess up and hit awesome you have to start your flawless bonus all over again.
You might need to play a few times to get the idea of when to hit the firework
just right.

Heck even I have trouble scoring flawless sometimes and I already
have my green car.
Then for the later autopia car quests that you'll need to do you should pay
a lot of attention to level 4 and 5's combo that gives you 80 points for level
4 and 100 points on level 5.
