Sound deaden your car and when you've finished do it again.

Doors could do with a layer or two on the outer skin (but inside the car) and seal the inner skin with another layer or two. We use dynamatt extreme over here and then spray everything you can't get the matt on. If you can't afford to do the whole car make shore you do the doors, large floor panels, roof panel, and next is the bulkhead, gear box tunnel, Boot/trunk.

I like to book customers cars in for two or three days strip the whole car down and give the engineer as many boxes of dynamatt as needed.
After that install the front speaker as far away from the driver as possible and facing the driver with tweeter close to the mid. I try not to put tweeters in the a-pillar. If the mids are in the doors try the tweeters in the kick panel. The tweeters must be facing to the listening position even if the mids aren't.
Next add sub system in the best box you can build for it with plenty of bracing to make it nice and strong.
Put car back together with modifications to make it all fit, sit back listen and adjust as needed.
As every one has said SQ comes from the install and then the equipment.