Use a razor, and get the best quality razor blades. Make sure that hair is thoroughly wet and well-covered by the shaving cream. (I also soften the hair with a conditioner beforehand.) Concentrate on a small area at a time. Stretch out the scrotum between two fingers of one hand, and with the other gently draw the razor across the surface. I usually go from the midline out to the side. You'll need to reposition the scrotum and continue with a slightly different area, and eventually switch hands for the other side. (Some degree of ambidexterity is advantageous.) In the beginning, don't worry if your shave is not perfect: so long as there's less hair after shaving than there was before, it's okay. In a couple of days you'll do it again.
I've never tried to shave my ass, so I have no advice for there; but as regards the crack, I have found that it's very irritating to try to shave every hair in the crease where the thigh meets the crotch, so I would go easy there. A little wisp of hair between two surfaces that rub together is not a bad thing.
New razor blades cut hair best; when the blades are dull, you're more likely to cut yourself. There's no need to press too hard against the skin; and scrotal skin is very sensitive anyway.
Finally, experience is the best teacher.
Best of luck,
D.
