So I can't really help much there... Although FWIW a quick glance of the doc you link to and it sounds like everything is pretty much covered... Have you set up some sort of temporary DNS (or added an entry to your hosts file?) so that your PC knows where to find these test domains? (and your server)

It's not really answering your question; but personally I find that editing the Apache config manually is pretty straightforward. I'm not sure what Webmin does though, so it is possible that it may be a little messed up after having used Webmin? Debian (and derivatives such as TurnKey and Ubuntu, etc) do it a little different to default Apache2 setup (and that used by Red Hat, CentOS, etc too) so if you go googling (for clarity or further info) then make sure that you keep that in mind.

You will find the configured sites in /etc/apache2/sites-available and the enabled sites listed in /etc/apache2/sites-enabled

To enable and disable configured sites (i.e. the ones in /etc/apache2/sites-available) use the a2ensite <name> (to enable) and a2dissite <name> (to disable).

I would recommend that you create a new site for each of your domains rather than editing the default site. You could use the default as a base though (e.g. "cp /etc/apache2/sites-available/default /etc/apache2/sites-available/new-site").