Often, not very easily.
Very basic principals:
It is certainly pointless applying a shoe onto horn that is folding under. The only way that it can grow is horizontally.
The heel has to be taken down to a level that isn't bent underneath. If the bar is curved and collapsed, I would reduce it until it was straight enough to actually give some support to the heel. The breakover at the toe has to be brought back. It is the flare of the foot that pulls the sole flat, so the flare must be reduced.
There is a problem, nowadays, about shoeing these flat feet with flares because most shoes are machine made (certainly in the UK) and the nail holes are too upright so that the nails don't pass a sufficient distance up the wall. It means that they are often shod out to the flare rather than trying to shoe to hold the shape. (I realise that this won't matter to all you barefooters!)
Sounds simple doesn't it!