Here is the overview on how Paint.NET plugins work:
Paint.NET Plugin Overview
There are a ton of examples of plugins on this page:
Paint.NET Plugin Examples
If you want more than just code examples, try this series of tutorials:
Part 1 - Simple Part 2 - Intermediate Part 3 - Complex Part 4 - Odds & Ends
If you really want to know the "bare bones" C# way of writing a plugin, load one of those examples into CodeLab, press Ctrl-B to build a DLL file, and click the "view source" checkbox before pressing the Build button... as shown in this page:
Building a DLL with CodeLab
This will show you the entire C# file used to build a Paint.NET plugin. You can select that source code and copy-and-paste it into a separate .cs file and open it in VS2005.