  We review and revise phenomenology of the GeV-scale heavy neutral leptons
(HNLs). We extend the previous analyses by including more channels of HNLs
production and decay and provide with more refined treatment, including QCD
corrections for the HNLs of masses $\mathcal{O}(1)$ GeV. We summarize the
relevance of individual production and decay channels for different masses,
resolving a few discrepancies in the literature. Our final results are directly
suitable for sensitivity studies of particle physics experiments (ranging from
proton beam-dump to the LHC) aiming at searches for heavy neutral leptons.
