The study of the highest energy cosmic rays is currently of paramount importance, due to the number of questions still without an answer. Where are they produced, what kind of particles they are and how they propagate to the Earth are some of these questions. A detailed simulation program could let us know beforehand the spectral differnces among different sources and the influence of the intergalactic magnetic fields in the measured spectrum. In this work a simulation program was created, including all physical processes that affect the propagation of cosmic rays. Using the smallest possible number of approximations and taking into account the most recent experimental data. Some preliminary results are shown, which prove the accuracy and capabilities of the program.