I am trying to remove a rotted sill plate, in a “non bearing” exterior  wall, and want to know how to shore up the wall, when in this case, there are no joists to shore under I say non bearing, but of course the rafters sit on top of this wall 20 feet up.

100 year old house, built backwards with joists and rafers bearing on all four walls.

I understand about removing several courses siding, and nailing a ledger on to the studs, and running abrace down from that, but what should be done in the ground, to hold this wal in place?