The best way to see the layout of a 'wizard.twiz' file is to view an existing one. Download a copy of the Presentation Assistant from the Tinderbox Public File Exchange, and unpack the DMG disk image. Ctrl-click or right-click the 'Presentation Assistant.pkg' file and select 'Show package contents' and in the new Finder window navigate down to the wizard file.
It is strongly recommended you use a proper text editor for this next task such as BBEdit or the (free) TextWrangler.
A wizard needs a minimum of 2 panels - a start & end!
Wizard file XML syntax and variables are described in separate notes.