1.i belive you need 8ga wiring. on the power (battery wire, all 8ga can be use as power or ground), connect it to the + of the amp, and make a run to the + of the battery. make sure you have a fuse with a fuse holder at most 9-12inches from the + battery.
2.if you going to put the amp in the trunk, lift the carpet to where you see the chasis of the car, and sand it to where you see straight metal. get some 8ga rings and crimp (since its a small wire, just crimp) and screw it the the shaved area of the chasis as your - Ground of the amp.
3. 18 ga will be the size you would need as your REMOTE wire on the amp. that wire will be connecting to the remote turn on wire behind the head unit you have. if its too complecated, just connected to the 12+ accessory wire (there is a red, yellow, and black wire together to my understanding. connect it to the RED WIRE)
4.then the RCA from your amp input to the Rca output of your head unit
5.then connect to the subs to each speaker terminal
and hopefully i gave you the right instructions on how to install an amp to your car