Elementary atomic structures of carbon are selected via the deposition of methyl radicals from the gas phase, and a kinetic scheme of the diamond crystal growth process is proposed. A macroscopic model of the growth process is developed in an average field approximation that corresponds to the kinetic scheme. The model is based on a system of homogeneous differential equations of high dimensionality that describe changes in the surface concentrations of elementary carbon structures in every crystal layer. Experimental data are in good agreement with results from simulations. The influence of different parameters and elementary stages of the process on the dynamics of crystal growth is investigated. The rate of one of the most important stages, downward migration, is determined.