I only just figured out and got the guts to do this.
You get your balls all contracted like its a bit chilly. not real cold thats too contracted.
You take your clippers, take off any long hair you may have on the sack.
Then you put shaving cream all over it and you hold your sack with one hand, gripping it a bit so the skin is a bit tight on the other side, then you can mach3 or gilette turbo or whatever other brandverb you prefer all over that shit, quite firmly even, without cutting yourself. even on the seam.