I just published a new how-to on OTN titled A Beginners Guide To Building Extensions for Oracle JDeveloper.
The reason I wrote this one is that I think a lot of people don't realize how easy it is to add functionality to JDeveloper. If you'll read this one and follow the code sample you'll see it is actually quite easy - especially since most of what you'll be doing is copy/paste from the extension SDK samples we provide.
So now you have the starting point to get you on the right track in your route to your first extension - try it out, it might prove to be fun...
Note - this is not a thorough explanation of everything you ever wanted to know about extension building. For this we have a complete on-line help chapter about this in the Extension SDK update you can download, and once you are done building your first sample you might want to explore that chapter in more details.