Redmine

Hi,
I need help, I'm new in redmine/rails development, and I'm totally lost.

The question is: if the user open a new issue in wrong project (one project per country), and before/after user save that issue, my created plugin must change this new issue's project_id automatically, that the new issue save to the correct country project.
I know the right country, because the form has a custom town field.

I read many-many tutorials, but exactly non of them gave a full, step-by-step answer.
I saw the before/after_save, but I dont't know, how can I use it in my plugin. How can read the wrong project_id from new issue? How can extend issue save methode?

Thanks!!!