The purpose of this tutorial is not to describe the theory of model fitting, but to focus on practical aspects and to learn how to work around difficulties. For practice, we use LITpro, a software currently developed within the JMMC 1 The web-site of the Jean-Marie Mariotti Center is http://www.mariotti.fr. 1 research group. LITpro is based on a modified Levenberg–Marquardt algorithm and its architecture allows a flexible implementation of complex models and fits with heterogeneous data. Through the analyzis of an example of a fit on simulated interferometric data, we learn how to deal with degeneracies and how to tackle the problem of local minima, with tools provided by LITpro.