If you are using a service for the subdomain, you will need to contact the administrator of the subdomain and find out how doing this works with their servers. There should be an IP address that might look like this:
72.14.253.147/~yoursite
You will need to configure the domain to redirect to that address, but in order to find that address, you will need to contact the provider of your subdomain first.
(Different servers = different systems = different configurations. We cant help you with a definite answer)
Also, make sure that its acceptable in your subdomain providers policy to make sure that this is acceptable on their terms.