This is how I did it.
I read few tutorials from people up there then I "walked" through code. Slim is really "slim", only ~50 files.
Check Slim.php file, everything starts there. See what's going on in the constructor, then see get/post methods. If you have xdebug installed then it could help you walk through one simple request and so on.
And as an added bonus you will learn a lot about PHP, patterns, basically how everything works.
P.S. My first answer here :)
