This primer accompanies the release of BAGS, software developed to calculate sediment transport rate in gravel-bed rivers. BAGS and other programs facilitate calculation and can reduce some errors, but cannot ensure that calculations are accurate or relevant. This primer was written to help the software user define relevant and tractable problems, select appropriate input, and interpret and apply the results in a useful and reliable fashion. It presents general concepts, develops the fundamentals of transport modeling, and examines sources of error. It introduces the data needed and evaluates different options based on the available data.