The radiative energy transfer (RET) theory allows an analytical calculation of the attenuation of a scattering medium, such as vegetation. Recommendation ITU-R P.833-5 details the RET-based model and gives the necessary input parameters for several tree species. However, if the vegetation is different to that found in Recommendation ITU-R P.833-5, one may derive the input parameters for RET from measurements. This Report details how to derive the RET parameters from a measurement.