The latest and more detailed installation instructions can be found here.
First install Python. Linux users should
have it in their repositories, Windows/Mac users can get it from the
Python homepage. You need the 2.7.x version (Python 3 is not yet
supported). Windows users, make sure to select the option to make
Python available in your path - this is so you can call it everywhere
as python. Python 2.7.9 and later also includes the
pip installer out of the box,
otherwise install this separately (in linux it's usually found as the
python-pip package).
Stand at the root of your new evennia directory and run
pip install -e .
(note the period "." at the end, this tells pip to install from the
current directory). This will install Evennia and all its dependencies
(into your virtualenv if you are using that) and make the evennia
command available on the command line. You can find Evennia's
dependencies in evennia/requirements.txt.