To replace the HU and use the MACH amps, you'll need to buy a FSA (factory system adapter) like this one.
Factory System Integration Adapter 1988-up Ford/Lincoln/Mercury/Mazda/Nissan at Crutchfield.com Any local stereo shop in town should have some on hand.
Replacing the door and deck speakers is pretty straight forward. Speakers are crewed in with a T15 torx. Once you unplug the speakers, you will need to cut off the factory speaker plug harness, strip a little off the ends and crimp some new terminals on there. Pay careful attention to the polarity.