"What if?" scenarios in urban digital twins are based on mathematical models and simulations. Being simplified representations of a real system, such models allow us to digitally experiment with various alternatives and make better-informed decisions for real urban development problems. Each simulation places us in "Model Land": that abstract and boundless land of mathematical relationships and calculations. Numerous decisions must be made to enter, navigate and exit Model Land towards reality. This "Guide to Model Land" is intended to be a practical guide to ethical issues concerning digital simulation models. It is divided into three sections: Entering Model Land, Navigating Model Land and Exiting Model Land. It was created based on extensive literature research and offers guidelines as well as references to further relevant literature.