What do you have so far?

The functions you will need are described here and you will probably need at least some of the functions described here. But from your message, I suspect you are not that far along yet.

Start here and tell us when/where you get stuck.

If you read that and your reaction is that it sounds far too complicated, it could be that you want to write a script, not a plugin. If you know Python, check out the Python Script plugin for Notepad++.