run power and ground wires from the battery to the (new) amps (fuse the power wire at the battery)
run RCA cables and a remote amp turn on wire from the radio to each amp
run speaker wire from the amps to each speaker (or sub) you are going to be connecting. for the four door speakers, you can either run wire directly to each speaker through the door boot, or you can run speed wire up to the radio (with the RCA's) and they use the factory wiring to get the sound to the door speakers
mount the amps down to something.
long story short, exactly like every other amp install.
PRO TIP dont waste time replacing the rear door speakers if you are adding new rear deck speakers, you wont really be able to hear them at all.
PRO TIP I would't do rear deck speaker, I would just remove them completely and put in good speakers in the rear doors, then there is a place for the bass to better travel into the cabin with out getting muffled by the rear seat, or if the subs move enough air, distorting the rear deck speakers.
PRO TIP the battery is in the trunk.