In the afternoon, you should take pictures of the longslit(s) you will use, in order to measure the CCD (x,y) target position you want to place a given galaxy/star. This images should be taken with the FILTER=CLEAR, GRISM=CLEAR, and APERTURE=longslit of your choice.
You should take images in both unbinned mode (1 x 1) and the quick binned mode (set binning to 4 x 4 in the ROI gui) . The latter has a quicker readout (30 versus 150 seconds), so for all but the faintest objects it is the setup imaging method of choice.
Display the images and choose a good part of the slit near the center to place the object. Use the IRAF task IMEXAMINE with option ``a'' to get a reasonable estimate of the (x,y) pixel value for the slit. Create a file with one line and the x and y pixel values entered on that line (separated by whitespace). Create different files for the unbinned mode and the binned mode.