Automatic print redirection is enabled by default, so if it's not working then you may have some legwork left to do. On the terminal server, try installing the driver for the printer that the user wants to print to. Try to avoid kernel-mode drivers, because a badly written kernel-mode driver will crash the operating system. (A group policy allows you to specify that only user-mode drivers be permitted.)
Also, make sure that automatic printer mapping is enabled. Check the client-side settings in Remote Desktop Connection (behind Options>Local Resources) to be sure that printer mapping is allowed. On the server side, check the user account properties or, for all remote connections, check the protocol properties to be sure that printer redirection is enabled.
This was first published in April 2006