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 (the standard START button at the top left of the COSMIC window) and the quick binned mode (the SNAP button at the top right of the COSMIC window). 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.