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 exisiting site.

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.

The configuration of Plesk is not easy and is specific to each plesk.

The rule described in the FAQ, describe the alias method to share the same directory than the master.
If you did that, you that mean that each time you enter a domain name, you reach the same environment. In this case, it is important to correctly spell the domain name in the JMS Multisites 'manage site" to allow detect the appropriate website. Pur the domain with WWW and without WWW in the definition of the slave site.

Sometime, this is just because you forget to define on of them that you get the master.

When the master website is displayed, this is because the domain name enter in the url does not match exactly the one present in the list of slave site.