The thing about transscrotal piercings is that you can't just clamp and pierce them with a standard barbell, so they're beyond what the average piercer can do. Doing them as a piercing risks trapping an infection (which won't be able to drain) inside the scrotum where it can rapidly (within a couple days) become life-threatening. I have friends who have nearly died from this piercing and other scrotal infections — take this risk seriously!
In theory, the “right way” to do this is by creating a larger hole or slot through the scrotum, and then the front and back are cleanly sutured together. This is the technique pioneered by Jon Cobb back in I think 1994 that is now used by virtually all professionals doing the procedure. Done right, the piercing can fully heal in under a month (and the larger hole has better drainage ability in the case of an infection).