It's a lot easier to learn out to install a head unit if you watch someone do it a few times, and then do it with them watching you. If you don't have a friend or relative who knows how to do this you can probably check YouTube for how-to videos. I can think of one video that isn't a how-to, but it shows some footage of assembling the wire harness and putting the aftermarket head unit into a dash.
skip forward if you want to roughly 10:00 to see when the head unit install starts