The answer depends a little upon your application but let us assume that your program objects are faint and are not expected to be readily visible on the rear-slit viewer itself. In that case, in addition to specifying the location of your program objects, you will also need the coordinates of two "setup stars". These stars should be bright (V=10-15), separated by 2-4 arcmin, and may be several arcminutes away from your actual program fields. Small square holes will be "drilled" in your multi-slit mask corresponding to the position of these stars, and when setting up on your field, you will first move the telescope to these setup objects and ascertain that the spectrograph rotation is precisely set. The telescope operator will turn on the automatic guider at this point and carefully center the setup stars. When all is well the telescope will be offset (and the guider moved an exact corresponding amount) to your program field. When exposing on your program field of course there will (hopefully!) be nothing but "sky" exposed through the setup star holes.
In addition to your setup stars you also may wish to have a "check" star within your program field. The purpose of this star is simply to ascertain that the positioning is perfect. The star needs to be faint enough (V>15, depending upon your setup) not to saturate during the exposure, but bright enough (V<17) to be readily visible on the TV.