A practical, illustrated guide to building local-level geographic datasets for sub-county policy analysis. Using Arlington County, Virginia and its 62 civic associations as a worked example, the guide walks through the boundary problem that plagues sub-county work, the data sources and methods for allocating tract- and block-level measures to custom local geographies, a parcel-based allocation approach, and the limitations analysts should keep in mind. It includes reproducible Python and R implementations of the full pipeline and a bivariate worked example mapping median household income against broadband download speed.