Graham bemethavinci Twig is pretty complex and really the only way to learn is to dive in and use it I am far from being an expert
the structure is very much like php
Xtgem uses version 1.16 this is the documentation http://twig.sensiolabs.org/doc/1.x/templates.html
I use the documentation and google to find examples as the documentation is quite brief on most of the commands
you need to start off with simple changes on a test site and keep backups all the time as you will make mistakes
if you don't want to get into the actual twig code at first you can just change the simple html that is embedded in the the script