When you create a new "Web Content Form" in Visual Studio 2005 and pick the master page to use.
Alternatively, depending on the type of web project it might be a tick box saying "Select master page" when you select "Web Form".
For a more general overview there are hundreds of articles on the internet.
e.g.
http://www.codeproject.com/KB/aspnet/masterpages.aspx
/- Sam Judson : Wrox Technical Editor -/