As I have resuscitated my neps multiple times from cuttings and basals, I feel I am experienced enough to comment on this.
What ChefDean said.
Wait until it is at least a couple of inches across. Cut it as close to the stem as possible. Put it in a pot of damp media and put the whole thing in a plastic bag. Give it bright, indirect light. As long as the leaves still look healthy, it is working on rooting. After a couple of months, adjust it to normal humidity. If during this time it starts to wilt, bag it and let it cook some more. Once it is able to survive without the bag, it is rooted.
The reason for the bag is twofold. First, it help to keep the plant and media from drying out. Second, neps are able to absorb moisture that it needs from the air through the leaves so proving a humid environment let's it get water without roots.
Panman's Grow List topic47056.html
Toes in the water, ankles wet, and knees damp.