Not too hard to get started. Once you have SNMPD installed and working just run:
apt-get install cacti
When that is complete you will have to modify the Apache config.
Add the follwing to "/etc/apache2/sites-available/default" between the VirtualHost tags:
Alias /cacti/ "/usr/share/cacti/site/"
<Directory "/usr/share/cacti/site/">
Options Indexes MultiViews FollowSymLinks
AllowOverride None
Order allow,deny
Allow from all
</Directory>
<Directory "/usr/share/cacti/site/">
Options Indexes MultiViews FollowSymLinks
AllowOverride None
Order allow,deny
Allow from all
</Directory>
Then restart Apache:
invoke-rc.d apache2 restart
Use the url http://hostname/cacti and log in with the username admin and password admin. You will be asked to change the password and then you're in!