Is almost the same as for an access database.
Go to your Datasources[ODBC] in Admin Tools in Control Panel. Select Add to add a dsn, then Where it asks you for a driver, go to the very bottom of the list and select SQL SERVER.
The rest is easy, just follow the Wizard, giving a name for your DSN, e.t.c
I would add it as a system DSN aswell, as some application don't seem to pick up User DSN's
-Richard