Green Screen Keying and Texture Overlaying

I'm not sure if this is the right place to put this (new to this "community" as well as film making), I want to start by saying !!THANK YOU!! to HitFilm! I've been pounding my head against the desk trying to figure out what I'm doing wrong with my green screen filming, adjusting lights, intensities, camera / subject positioning, etc., etc. Always coming back looking like total crap, (I was using Sony Vegas 11). I purchased HitFilm 2 Ultimate a few weeks ago only because I wanted to play with special effects, on the verge of giving up on it, last night I decided to try it again using HitFilm, took a short test clip, and voila!! Using HitFilm it looked nearly flawless and I can SEE the minor adjustments I need to make to improve it.
I'm currently working on just a short vlog to put on my YouTube channel to hopefully perfect it.
Also, can someone point me in the right direction on the how-to video for applying textures to a subjects skin, faces, etc. For example, overlaying a stock image of cracked dirt onto a subjects skin to give the cracked / flakey appearance. I've been looking and haven't seen one on that, I'm probably just missing it.