System assembly questions

Not too hard for cable management. Just depends on how clean of a look you want. Depending on your case you will be able to do a better or worse job. Better it is the better the airflow is and if you have a side window it looks neat.

For the GPU installation you first have to remove the back expansion ports that the GPU will fill. You then install the GPU into the PCI-E x16 slot and screw the card to the case on the back expansion slots. Lastly hook up the power from your PSU to the GPU. Then when you power on go the the GPU manufacturers site (AMD or NVidia) and download the latest drivers.

I built my PC yesterday and I just followed the New Egg video, Best thing to do is to refer to your GPU manual which should show you pictured instructions on how to install it (It's really easy) . For cable management I improvised and was the most time consuming part of the build for me.

Edit "He installs the GPU right near the end of part 2 "  

Read the manual, yes.
Cable management is an art, not a science. There's no basic instructional video. I don't know what your case is, but if it has a cable management space (behind the motherboard, generally), your goal is to have as much cabling in there as possible and as little as possible out in the open space of the case, where it will disrupt airflow.

I've a cooler master haf 912, It's big so I think I can do a decent job, well all this cable management be in the manual or can you tell me what direction typically should the wires be going you said behind but like south?