How do you create your own drag-n-drop GUI designer ?

Question

Hello

I was looking at creating a UI for developing web forms, similar to the Netbeans Visual JSF form designer.It will be targeted to use GWT/GXT components.I am looking at a look very similar to VS/Netbeans.

Any thoughts on where/how to start ? Initially, I would prefer to have it as a standalone application, later develop it as eclipse plugin.

I have already evaluated the extjs designer and the eclipse plugin from instantiations, but i would prefer to have it developed on my own, as it looks challenging, Also, I have few custom components that have been developed.

Thanks