creating sub-domain clients

[Vullcan]

so you know what i mean i will give examples.

#1 account: uses v-gaming.org domain.

#2 account: uses sub-domain cadaveria.v-gaming.org.

can someone please explain how to do #2 please? i can only seem to make a sub-domain thats not a client account, but some kind of ftp account or something.
ive only been using a dedicated server, plesk, and windows 2003 for 1 day, so please keep it simple.

thanks