A good guide to iDVD 7 can be found here:
Apple's tutorial on iDVD 07/08 is here:
Previewing before burning:
For user Library files:
NOTE: In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
To change the default music for a theme's menu bring up the Info pane for the menu, type Command+i, find the track you want to use in the4 Media/Music pane and drag it into the Audio bin of the Menu Info window.