SubDomains, Folders and One-Clicks - oh my!

Thanks for the replies on my earlier post (Just don’t get it)! I have been playing around in Joomla and am understanding CMS better. What I’m having trouble with is Dreamhost: when I go to add a subDomain, I don’t understand the step-by-step. What I thought was right, at one point, ended up adding an extra folder to my subdomain.

Same for One-Clicks. Everyone makes it sound like you just hit install and it goes. But I don’t understand where I’m pointing it to and what to fill in the blanks. When I think I’m correct, I get an email that tells me my directory was not empty. But I thought I was creating a new directory.

What I really need is a step-by-step, and I don’t see that clearly defined on the Wiki, unless I’m overlooking it. btw - Thank you to everyone for your patience with newbies! From all the replies I’ve read, you all are being extremly helpful!