Models are important in science to help us conceptualize better the interactions in systems. In the case of sustainable development, first thing is to identity the components of the system, some of which are: population size and age distribution, resources (soil, rainfall-annual average, trends, crops, fertilizer-source, cost, fuel-type, availability, other non-renewable resources, renewable resources etc. These need to be related in the model to aspects of use and reuse, transportation costs, storage costs, distribution, etc. I suggest you take these variables and others you decide are important and create a diagram or concept map relating them. That would be your model.