You create your search scopes from within the SharePoint UI. Click Site Actions, Site Settings, Modify All Site Settings.
In the Site Collection Administration column, select Search Scopes. Click the new scope button and create your new scope.
On the View Scopes page, click on the scope you just created and click Add rules and set the rules for your scope. After you are done with your scope rules click OK.
On the View Scopes page you will see a status that indicates how long it will be before the scope will be available. The only way you can force the scope compilation job to run without waiting is to go into your Shared Services Provider within Central Admin and manually start the update.
- Ben