Relativistic corrections to the evolution of structure can be used to test general relativity on cosmological scales. They are also a well-known systematic contamination in the search for a primordial non-Gaussian signal. We present a numerical framework to generate RELativistic second-order Initial Conditions (RELIC) based on a generic (not necessarily separable) second-order kernel for the density perturbations. In order to keep the time complexity manageable we introduce a scale cut that separates long and short scales, and neglect the “short-short” coupling that will eventually be swamped by uncontrollable higher-order effects. To test our approach, we use the second-order Einstein-Boltzmann code SONG to provide the numerical second-order kernel in a ΛCDM model, and we demonstrate that the realisations generated by RELIC reproduce the bispectra well whenever at least one of the scales is a “long” mode. We then present a generic algorithm that takes a perturbed density field as an input and provides particle initial data that matches this input to arbitrary order in perturbations for a given particle-mesh scheme. We implement this algorithm in the relativistic N-body code gevolution to demonstrate how our framework can be used to set precise initial conditions for cosmological simulations of large-scale structure.