My proposal for selected Euro exit recommended an overnight change to all bank accounts following hasty and secret week-end meetings to trigger the exit. It does not allow advance printing of new notes, as this would alert people to the coming changes and trigger a run on the banks in the affected country.
I suggest that new drachmas or escudos be issued at the exchange rate of one drachma or escudo for one Euro. They would not, of course be worth one Euro, as the new currency would devalue as soon as it traded against the Euro. That would be one of the main purposes of issuing a new currency, to allow a devaluation to aid adjustment of their economies to the competitive realities. The one to one rate would be penal exchange rate for all those holders of Euros in the exit country that you decided had to take some of the losses resulting from the economic failure.
Holders of Euro banknotes outside the EU would be unaffected by the exit of say Greece. They would still have Euros the day after the exit. Greek citizens would be expected to switch their Euros at the one to one rate into drachmas. If they turn up at the shops the day after the creation of the drachma, their Euro notes are accepted as drachma notes pending the issue of new drachma notes to them at the one to one exchange rate. If a foreign non EU tourist turns up in Greece with Euros they would be able to go to a foreign exchange shop and get an enhanced number of drachmas at the market rate as a normal foreign exchange transaction.
During the transitional period some Greeks would be tempted to take Euro notes out of the country and to switch them into other hard currencies. The scope for this would be limited, as from the moment of exit and the creation of the drachma any money they withdrew from their bank accounts would be drachmas. They would no longer be able to withdraw Euro notes. The notes they currently have would not be allowed out of the country legally, and would be subject to whatever policing arrangements the Greek state wanted to impose to try to prevent cheating.
The issue to be resolved for the EU is the treatment of Euro notes held by non Greek EU nationals working or living in Greece. The simplest way forward would be to allow them to keep their Euros and to exchange them at market rates when needed into drachmas. The EU might, however wish and be able to assert its jurisdiction to make defined categories of people suffer the loss on their notes as Greek citizens would do.