Scleral Depression is an art form that gets better with time. I first noticed my scleral depressor during an actual exam during my 2nd year. It is really disorienting at first though. Here are some tips:
Try to scrub in on some retina cases and ask to depress during endolaser or other parts of the case. The patient will be anesthetized, so it will give you an idea of where to place the depressor and how hard you have to push to see the desired effect.
Second: Pick a stoic old guy to try it on first. Give them a drop of proparacaine, and go on top of the lid rather than under it. Also, when trying to get the hang of it, I think it is easiest to see the superior retina. Make sure the patient is reclined. Also, use your non-dominant hand for the depressing, because it is all for naught if you can't get the 20D lens in focus. Hope that helps!