Tired of the battle, please help.

OK, so I have now been battling with this system for some weeks, and I still cannot get it to do what I want in a manner I feel is normal.

Heres what I am after.

Web hosting.
Mail hosting.
No problems so far.

Now I want customers who have there websites hosted on my server to be able to access there email via a webmail client (roundcude/squirrelmail).
So I set things up as mentioned in articles found on howtoforge. That doesn't seem to be working for me.

Problem:
The way it is configured clients don't seem to get access to their email via their own site.
IE. if there is a site www.mysite.com then the client should be able to access their email at webmail.mysite.com

I also would like this to be setup automatically and not have to manually set it up every time a new site is setup.

Please forgive me if I have this all wrong, but i don't have to many smarts when it comes to this sort of thing, and I have read so much stuff found on the Internet that my head is probably so full of irrelavent BS that it isn't helping.

Simple instructions like;
Install ISPconfig.
Do this.
Do that
enjoy

Are the best instructions.

Any comments would be most helpfull.

Regards

Fred