First of all, if you want to implement user authentication, I would advise you to read this article first. You may find that you don't need to extend RequestProcessor in order to do what you want to do.
If you do decide to extend it, here's the process
1-Download the Source code for the version you are using.
2-Study the methods in RequestProcessor, get a feel for what they do, and identify the method that handles the behavior you want to modify.
3-Write your class extending RequestProcessor and override the method. If you want to add logic to the method rather than replace the method, make sure you call super.theMethod() in your overriden method.
4- Register the class as the request processor for the mudule by inserting the following tag in your struts-config.xml file: