http://www.ign.com/faqs/2004/fire-emblem-random-number-generator-faq-520430

This is an old af guide about RNG manip that does a good job in showing how one would actually go about using it casually, and explaining some of the basics. Not all the specific information is accurate, so if you find something that conflicts with the comprehensive RNG guide on this site, the comprehensive guide is right.