install Python, version 2.0 or later - http://python.org/

install wxPython - http://wxpython.org/

(under debian, you can currently get the above using 
apt-get install libwxgtk2.2-python 
from testing and use python 2.1)

untar and run: python setup.py install

put a line in /etc/mailcap which is similar to the 
following, only replace the path to 
/usr/bin/btdownloadgui.py with the one it's actually in.

application/x-bittorrent; /usr/bin/btdownloadgui.py %s; test=test -n "$DISPLAY"

You may have to restart your web browser for it to start 
using BitTorrent.

If you're using a web browser which doesn't respect 
/etc/mailcap you can go into the mimetype configuration for 
your web browser and manually associate application/x-bittorrent 
with btdownloadgui.py (with the appropriate path, of course.)
