A good guide to iDVD 7 can be found here:
Apple's tutorial on iDVD 07/08 is here:
Previewing before burning:
First make sure you have iDVD installed on your Mac. If your Mac came with Lion preinstalled then you will not have iDVD as it has been discontinued by Apple and is no longer supported or offered. If that's the case you'll have to purchase an iLife 09 or 11 disk from a 3rd party retailer like Amazon.com or eBay.com.
However, iLife 11 only provides iDVD 5-7 themes. The Software Update no longer installs the earlier themes nor do any of the iDVD 7 updaters available from the Apple Downloads website contain them.
Currently the only sure fire way to get all themes is to start with the iLife 09 disc (Amazon.com: iLife 09):
This shows the iDVD contents in the iLife 09 disc via Pacifist:
You then can upgrade from iDVD 7.0.3 to iDVD 7.1.2 via the updaters at the Apple Downloads webpage.