WPF Custom Designer - Where to start?

I know that there are similar questions but this is a little bit different.

I'm trying to write a Custom WPF Editor using Visual Studio's WPF Designer capabilities.

What I think to do is an isolated VS Shell Application.

Following the tutorial: http://msdn.microsoft.com/en-us/library/bb685514(v=vs.110).aspx  I was able to create a simple isolated Shell that do nothing.

Now the question is: What are the next step to include a WPF Editor inside my VS isolated Shell, load and edit Custom User Controls?

Thank you