Unless you have a rear set of RCA aux in (most HUs do) then you will have to use the front aux in yes.
You are right about the amp power on wire from the HU to the amp this fires up the amp, usually blue/white but depends on the HU
You dont need anything to run from the amp back to the HU just the RCAs from the HU and a power on, and obviously a power supply from the battery.
If you are not sure why not just pay someone to sort it all, no shame in that