The author presents basic concepts in modelling, using the example of coupled chemical reactions. Four points are examplified: the necessity of modeling assumptions, the need of a multiscale analysis in order to cope with very different characteristic times of the system, a good understanding of deterministic versus probabilistic source of the models involved and finally a practical understanding of computational algorithms by using simplified codes. After introducing the key concepts in multireaction chemical modelling, the master equation method is explained and stochastic simulation algorithm is shown to be computationally efficient. Finally numerical results of simulations applied to a simplified model of reaction rate equations are presented, including a MATLAB code.