There's really no skill involved with defacing. There's a lot of different ways...if you have ftp access, you can del the index page and replace it with your own. You can also use a web shell to del/replace with your own index page, or if the shell is fancy enough, you may be inclined to simply edit the index page from the shell. If your only attack vector is sql injection, then you can try an into/out file query. Keep in mind, it doesn't have to be the index page, but that's generally the one that gets defaced.
I dont know man...it's really not that complicated, and there's a million different ways to do it. Don't get hung-up with learning about it, because it's honestly a huge waste of time.