I have read the guide, followed some videos, read some forum threads and am NONE the wiser. It's all very confusing. Is there a laymans guide to getting ONE slave site working...
ie. a quick, multi-step guide.

I have my own servers and am not a newbie to Joomla but I cannot work it out - Joomla is installed 1.5.20 with Multi-sites and I have added in Multisites a "slave" domain. But then I'm stuck. How do you get the slave to connect/share the db -

if I setup the domain aliais as per instruction (for Plesk) then all I get is a replication of existing site and no possibilities for creating new/different content.

Please help - I think this could be excellent solution and maybe I am being thick but this seems impossibly complex - so I am obviously missing something. Can you help please.

A very basic website creation is the one described in the tutorial video 9.
It creates a slave site into a subdirectory that works with Plesk.

The problem with plesk is the security that is added by plesk when creating domain and subdomain. To open the security, refers to the Plesk help - appendix A that explain how to modify the vhost.conf for each domain and subdomains.
If you don't want to have speicifc domain or subdomain, the alternate solution is to create synonym with aliases in Plesk so that you have several domain using the same document root directory.

The complexity is the Plesk configuration and unfortunatelly we never found a generic description for Plesk. Each time we did a billable support to configure Plesk that was different.