I'm in zone 5 (Southern Ontario), and my family buries the fig trees for the
winter. The oldest are probably about 20 to 25 years old (maybe more). The
process is as follows:
- use twine/rope to gather branches together (as tight as possible without
damaging the tree)
- dig a trench from the root ball long enough to accomodate the height of
the tree
- dig around the root ball (using some water from the hose, if necessary) to
loosen the ball from surrounding soil
- tip tree into the trench
- cover with burlap/tarp/plastic then backfill soil from trench, adding
leaves and other yard waste for insulation.

I was always warned that you needed a good seal to prevent rodents from
seeking refuge and nibbling away at your tree, but in a half dozen years,
I've seen no evidence of any such damage.

Andrew