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Basically we do not have any best practices how to use a certain Silverlight control in Sitefinity. It would be the same as to use a Silverlight control in an ASP.NET application. You can a look at the RadCoverFlow demos
and RadCoverFlow documentation
to see how to develop with the control.
What you should do is to add a Silverlight application to your project, and achieve the functionality that you need from the control there. Then in on your Sitefinity website part of your project you will create a user control that will wrap in the Silverlight application. An example how to do this can be found here - Using Telerik Silverlight controls in Sitefinity
the Telerik team