No expert by any means, but here's what I've done with lots of success. I'll brush off any big dirt clogs attached to the onions, but I don't really try to clean them up to much. I'll lay them out in a shady spot in the yard for the rest of the day if it is nice and dry that day. I then bring them in doors, and place them in a single layer on my elevated drying rack (an old baby gate). About a week and a half later, the necks have completely withered and the greens are a dry brown. Then I remove the greens and brush off any surface dirt that comes off easily. I end up with this:
