Changing forum component URL to *.example.com

[PhysXX]

I have tried following the manual provided by vBulletin but I just can't get this to work somehow..

Long story short:

WordPress is installed at root directory ("public_html")
vBulletin is installed at ("public_html/forum")

So whenever people go to "www.mydomain.com/forum" they end up at the forums.
I want to change this to "forum.mydomain.com"

Can someone please provide me with a logical step by step guide on how to perform this, please :erm:

[ShawneyJ]

after you add the sub-domain forum.mydomain.com in your cpanel, all you need to do is add http://forum.mydomain.com into your vbulletin options>> Site Name / URL / Contact Details>>Forum URL and refresh. i cant remember having to do anything else but that.

add Subdomains in cpanel to point to forum folder forum.mydomain.com.
then update your new Forum URL in admincp options.