Build for TP-Link Archer C1200-AC1200

I would be amazingly thankful for at least minimal guide how to properly build GPL sources. (I had some experience with gcc and I'm linux user). And how you're making changes to busybox, because I want to make at least the same for router Touch P5, which as I know is similar to this one.

There is some information that I've got from it, maybe you'll have any ideas what to do next

Thanks in advance.