Put the respective lines into your Emacs initialisation file:

(add-to-list 'load-path "PATH/TO/PYTHON-MODE/") 
(setq py-install-directory "PATH/TO/PYTHON-MODE/")
(require 'python-mode)

To change the Python default shell use 

M-x customize-variable py-shell-name 

or write 

(setq py-shell-name "MY-PYTHON")
resp.
(setq py-shell-name "PATH/TO/MY-PYTHON")

IPython users for example may insert "ipython" with
M-x customize-variable py-shell-name 

or in init-file

(setq py-shell-name "ipython")
resp.
(setq py-shell-name "PATH/TO/ipython")
