All non-virtual footprints must have 3D model references, even if the 3D model is missing (does not yet exist). This allows the 3D model to be added later without requiring the footprint to be edited again. (It is not required to supply a 3d model file.)

3D model files must be placed in a library (directory) which has the same name as the footprint library, with the extension .3dshapes

3D model files should be named the same as the footprint (ignoring file extension)

If a footprint is a simple variation that does not change the 3D representation, the common 3D model should be used (do not duplicate models unnecessarily)

Model scaling must be 1:1:1 (model file should not require any scaling to correctly fit the footprint)

Rotation and offset must be 0 (The model must be aligned correctly to the footprint within the 3d modeling software.)

Path to 3D model must have the ${KICAD8_3DMODEL_DIR}/ prefix

Filetype must be .wrl (KiCad replaces it with .step automatically for the step export.)