Installing Py6S itself is very easy (
pip install Py6S for those of you who know what that means), but unfortunately compiling and installing 6S itself is a little harder. Don't worry though - there are thorough instructions which take you through compiling and installing on 6S, and then installing Py6S on Windows, OS X and Linux. Py6S is free software, available under the LGPL license, and the full source-code is available at Github.
The quickstart guide takes you through your first steps with Py6S. You might also find the Py6S journal paper useful, as it has various examples in it. Please note, you must cite this if you use Py6S in any academic work.