This guidebook is an introductory tutorial on the use of mathematical modelling (mostly Monte Carlo methods) in electron beam processing. It starts with an electron beam processing background presentation, and provides a short introduction to mathematical modelling in general. Typical irradiation problems are also addressed in examples with solutions which the reader can follow. The target audience of this guidebook is readers who have a basic understanding of electron beam technology and who want to evaluate and apply mathematical modelling for the design and operation of irradiators, as well as those who seek a better understanding of the irradiation process and process development for new products.