Optical interferometers are instruments that combine the light coming from separated optical telescopes in order to get information with the spatial resolution of the telescope array. One of the major interferometer is the Very Large Telescope Interferometer (VLTI). To observe an astronomical target with the VLTI, like with any other interferometers, one needs to carefully prepare the observations because of the numerous parameters to be fixed. In addition, the VLTI is being developed in the framework of the data flow system created for the Very Large telescopes (VLT) at the European Southern Observatory (ESO). I recall the philosophy of the ESO/VLT data flow system and its adaptation to the VLTI data and then I describe the various steps required to prepare a VLTI observation. Finally, I illustrate the various steps of this preparation with an example focused in the observations of the young binary system Z CMa.