Below is a link to a primer showing how to write a very simple von Mises plasticity model using the classical radial return method.  Highlighted in yellow you will see an important warning about the limitation of such models. Nevertheless, this primer is a good place for a beginner to start. Also, the last two pages of this primer describe two very simple verification tests, which anyone who runs a plasticity model in a code should always test first.

Tutorial On J2 Plasticity