We here present a manual for the reduction of data from ionograms obtained from the Mars Express MARSIS Active Ionospheric Sounding topside radar sounder. Sample data are presented with the procedure for processing them explained as simply as possible. We discuss the uncertainties inherent in the measurements as well as systematic problems with the data. A sample code is included to facilitate the inversion process. We also include a comparison with an electron density profile taken from the Mars Express Radio Science occultation experiment, showing agreement between the two methods, although the data are not simultaneous.