If you are new I do not want to even get into the kicks. It is extensive work & with out prior knowledge of welding or fiberglasing I would only loose you on a forum.
Without extensive knowledge the 1/4 panels might also be over your head. But hear is a link for you...
http://www.carsound.com/UBB/ultimate...c;f=4;t=006819
It is not a Mustang & it is not a sub, but it gives you a basic idea of some work involved.
Here is a Honda Civic with subs in the 1/4 panels.
http://www.cardomain.com/ride/150903