Webgen is a static website generator. Easiest is to start with an existing webgen example site, or a default site generated by "webgen create", then:
There are a few useful details and maybe a couple of gotchas along the way, but that is basically it.
First gotcha: as of this moment, there are two webgen packages in the Debian repository: webgen and webgen0.4. If you start with a webgen0.4-generated site from somewhere else, and then install just webgen, which is currently version 0.3.8, webgen may fail with some rather obtuse errors. Be careful which version you are installing, where your example site is coming from, and which version was used to generate it.
The list of templates is actually a little hard to find on the website, so here it is. To install a new template named "andreas07", in the site root (one directory up from the src and output directories):
webgen use website_style andreas07
Note that webgen will not overwrite existing template files, so you will probably first have to do a "rm default*" and "rm *.css" in the src directory.
Then use your favorite text editor to tweak src/default.template and src/default.css to make the site look exactly how you want it. Graphics used in the layout are found in the images directory, and of course may also be customized.
Check your layout by running "webgen" in the site root (one directory up from the src and output directories) and point your web browser at the output directory to see the result. Note that webgen runs very fast, so it is a good idea to frequently check your layout changes so that the cause of an error is immediately obvious.
Once you are satisfied with the layout, start adding content to the src directory. Each *.page file corresponds to a page on your site, and after running "webgen" will appear as a *.html file in the output directory. Each page will also appear in the site template's output HTML menu.