How to build YOUR OWN template in iWeb - Revisited
That’s why I stick VERY closely to a combination of the techniques I described down here. I change manually what I have to do and then start building things in iWeb for a template.
In practice that means that I change:
fonts
fontSize
all navigation-bar stuff
defined width and height sizes for items like navbar, header, footer, main-layer, navigation-bar etc.
style-sheet definitions for various items like the buttons etc.

Quick & Dirty Procedure
Start from scratch. Non-existing domain.sites file.
Create 6 new pages from f.e. “White”.
Name the site - this will be your template-name. Save.
Quit iWeb
Run site2template to create a new base-template set. See the instructions on how it works: drag&drop site2template on Textedit.
REPLACE the index.xml.gz in every .webtemplate package (use the originals - in this example I’ve used “White”)
Unzip (double-click) all index.xml.gz files
Delete the .gz files
MODIFY the “TemplatesInfo.plist” file located in /Applications/iWeb/Contents/Resources/English.lproj/Templates to have your entry reflect the same model as f.e. “White” - i.e. the version entry (1.1.1 only).
Copy f.e. the “Welcome” page to the “Blank” folder and rename it to Blank (1.1.1 only).
Start the manual edit of all entities.

Yes, that works. You can leave the files unzipped. And test what you’ve been doing on-the-fly. I never quit iWeb when changing stuff.  Change, new page, change, new page... etc.
Yes, you can build your own!
And it’s not so very difficult either. (UPDATED)
 
if you follow all the steps here, you should end up with a template you can use to automatically include everything you want. Beware, it’s easy to FUBAR things.
 
Every single template(set) is unique.
Although some of the variables may stay the same and are recognisable, the style-names used for them differ from template to template. Comparing “White” and “Freestyle” revealed a lot of differences. Regardless, the mechanism stays the same, a “navbaritem-style-rollover-default” will be found in all templates.
 
You can use “site2template” too
(internet search or macosxhints.com). This DOES make things easier. Keep in mind that some pages like podcast and blog are easy to fubar, meaning they won’t work anymore like you expect them to. Also, a lot of the header-stuff is different. Short story: IMHO a quick way to get a new template,
 
Note on site2template: Don’t do it twice.
What do I mean? Let’s say you created a site containing all 6 pages and are ready to create a template from it. Great!
Let’s say you use the created template and rebuild things to become just another template. Don’t.
With hindsight I think that’s the exact point where iWeb went fubar the last time. Showing up with an empty template-chooser.
 
(Likely cause: (invisible) damage to the TemplatesInfo.plist file HAVE A BACKUP!)
 
Old tutorial
The old tutorial containing loads of information on the extensive procedure and contents of the XML files is still here:
Stuff has been trimmed down. Moved around. It will take some time to complete it all, but it starts here. The tutorial to create your own template(s), step by step. The Q&D way is still on this page, for those who want to use it. Nothing wrong with it.
Since it does contain some tips & tricks... and you’re here anyway... why not read it? ;-)
Topics
This is a set of topics I think could be of interest. Once they turn blue, they’re on-line :-) This could take a “few” weeks before things are there the way I like them.
I might do requests... if I have the time. Just ask. If there’s a suggestion, let me know
Create NavBar images
Debug & modify Templates
Customised NavBar
Unlocking Templates
Add YOUR pagecounter
Old Template tutorial
Resources
Remark/suggestion
Mail...
Just remove “move” from my email address. It’s to prevent robot-collected unwanted stuff. Sorry for the inconvenience.
Creating an empty template