In the discussion which follows, we will generally assume you are using the tip-tilt camera. However, most of the procedures apply to either guiding system.
One of the greatest difficulties in IR spectroscopy is precise centering of an object in the slit. If the object is brighter than about V=18.5 (depending on the lunar phase and the seeing) it is probably easiest to try and center on the optical image in the tip-tilt camera in panoramic mode. (This camera is available even if you are guiding with the offset guide probe.) It is important to realize however that crosshair on that monitor refers to the center of the chip which is carried on a moveable x-y stage. It is essential that a.) the stage be in the "middle" position and that b) you have previously set up the middle position so that it corresponds to the center of the IRS slit.