Except for the 30 second polling, this is pretty standard. Read the manuals here: http://www.cacti.net/downloads/docs/html/faq.html
- Using Cacti
How do I create traffic graphs?
Now that you have working SNMP
support, follow these steps.
Click Devices on Cacti's menu.
Click Add to add a new device.
Type a description, hostname,
management IP, and SNMP community.
Make sure to select "Generic
SNMP-enabled Host" under Host Template
and click Create.
Click Create Graphs for this Host at
the top of the page.
You should see one or more interfaces
listed under the Data Query [SNMP -
Interface Statistics] box. Place a
check next to each interface that you
want to graph. Below the box, select
type of graph that you want to create
(bits, bytes, summation, etc).
Click Create, and Create again on the
following screen to create your
Normally Cacti (and MRTG) poll every 5 minutes. If you want to make this more aggressive (make sure you understand what and why you are doing this) you can change the polling service to run as frequently or as infrequently as you wish. My suggestion would be to start with the standard polling interval first.