This document aims at providing the ASTRI (“Astrofisica con Specchi a Tecnologia Replicante Italian”) scientific community a detailed description and recipes to simulate the spectrum of a HE/VHE astrophysical source with ctools, in a way that is easily parallelised. In particular, the spectrum is divided up into several energy bins and simulated within each bin with the tasks ctobssim and ctlike. We also discuss the need for multiple realisations in order to obtain the most representative simulated spectrum of the source and how to treat upper limits.
