The amorphous high-temperature ceramic a-Si3B3N7 can only be synthesized via a sol-gel process. Here, we present a separation of time scales approach that can be used to model various aspects of the sol-gel process, resulting in a model for the synthesis as a sequence of dynamically well-separated steps. For chemically reasonable choices of process parameters in the actual implementation, we find satisfactory agreement with the structural data of the amorphous ceramic, in particular regarding the medium range order observed in NMR-experiments and the overall density.