Shave in the shower after you've done your normal cleaning routine, that way the hair and skin are softer. I only shave my balls because I prefer how it feels for me, don't really care if it robs me of any masculinity, I'm a liberated man.
Don't do it just before play, do it the day (or at least a couple hours) before just in case there's any mistakes. I don't go over any one area too much because razor burn is a real pain down there. So, if I miss a few hairs I get them next time.
If you're planning a larger area for shaving just as with your face go with the direction your hair grows, not against it.
Be very careful and certain you want to shave your ass hole if that is part of the plan. Ya, it feels great but when the hair grows back it is most uncomfortable. Once, just before a date with 2 tops I decided to shave my ass, very carefully, under the bath tub spout, was perfect, then carelessly got up to fast and gouged the top of my ass on the spout. Had to cancel last minute and then suffer the growing in of the hair for nothing.
I personally don't like having someone else do it and don't like doing it for others. Start with a small area, see how it goes, a mirror might help but I usually don't use one because I get confused with the reverse technology. Oh, shaving oil works great down there as well.