Easterling et al. (2000) originally proposed a size-structured IPM as an alternative to matrix projection models for populations in which demographic rates are primarily influenced by a continuously varying measure of individual size. That model was deterministic and density-independent, analogous to a matrix projection model with a constant matrix. Nothing could be simpler within the realm of IPMs. In this chapter we use that case to introduce the basic concepts underlying IPMs, and to step through the complete process of building and then using an IPM based on your population census data. To illustrate the process of fitting an IPM to population data, we generate artificial data using individual-based models of the monocarpic perennial Oenothera glazioviana, and of the Soay sheep (Ovis aries) population on St. Kilda (Clutton-Brock and Pemberton 2004).