Customizing an SharePoint 2010 Application Page.
In this example I'm customizing the /_layouts/MWS.aspx, the new meeting workspace page.
These steps can be followed for any of Application Page.
1. Create the custom application page and deploy it into the _layouts folder.
2. Create an IHttpModule to redirect all requests that come to MWS.aspx and redirect them to customMWS.aspx:
3. Add a new element in the web.config configuration/system.webServer/modules section to enable this IHttpModule. To do this create an web application feature to add this change in the web.config of the application.