I take it you mean "How does ONE connect to another desktop using remote assistance"
Once the PC's are networked or a VPN is setup, you can enter the computer name\path in the connection box. You'll find this by Start >>>> All Programs >>>> accessories >>>>> communications >>> Remote desktop connection
Follow the instructions there, and select the Pc you want to Connect to... and you should be all set.
in addition to Persons answer - the pc you are trying to connect to must have 'remote desktop' enabled.
you will find this by right clicking on my computer and going to properties.
the you click on the 'remote' tab and tick the relevant box.