  Spatial agent-based models are increasingly used to investigate the evolution
of solid tumours subject to localised cell-cell interactions and
microenvironmental heterogeneity. Here we present a non-technical step by step
guide to developing such a model from first principles, aimed at both aspiring
modellers and other biologists and oncologists who wish to understand the
assumptions and limitations of this approach. Stressing the importance of
tailoring the model structure to that of the biological system, we describe
methods of increasing complexity, from the basic Eden growth model up to
off-lattice simulations with diffusible factors. We examine choices that
unavoidably arise in model design, such as implementation, parameterisation,
visualisation, and reproducibility. Each topic is illustrated with examples
drawn from recent research studies and state of the art modelling platforms. We
emphasise the benefits of simpler models that aim to match the complexity of
the phenomena of interest, rather than that of the entire biological system.
