1. Printing drachmas?

What actually will happen if Greece leaves the euro zone and goes back to its own currency? How would that work? Is there a printing press somewhere busily churning out drachmas just in case? Or did they keep the old ones in storage? How will Greeks get new drachmas? Will they exchange their euros for them? How will the exchange rates be determined? How will all the software for cash registers and credit card and e-commerce transactions be reprogrammed?