Interfaces between dissimilar materials control the transport of energy in a range of technologies including solar cells (electron transport), batteries (ion transport), and thermoelectrics (heat transport). Advances in computer power and algorithms mean that first-principles models of interfacial processes in realistic systems are now possible using accurate approaches such as density functional theory. In this 'quick-start guide', we discuss the best practice in how to construct atomic models between two materials and analysis techniques appropriate to probe changes in local bonding and electronic band offsets. A number of examples are given related to perovskite solar cells.