I've never used subdomains before and I'm not sure how to do it I can't see a subdomains link in the Domain Monster control panel.

You need to login to the domain you want your subdomain on and then create an A record in the Manage DNS page. Hostname = your subdomain, IP address = the IP of the place the website you want hosted on the subdomain.

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Now I know how Joan of Arc felt,
when the flames rose
to her Roman nose
and her Walkman started to melt.