Help with installing amp!

I went to an audio shop and they quoted me $400 (including amp) as a minimum, I dont have that much money. I also don't know anything about wiring.
They said they would take a 4 channel and run it through the stock amp.
Ive looked and coundnt find any pictures or video on a DIY on how to do it. I could have missed it, but If someone could help me out please do so.