The common answer to this question is that Greece can’t do it. But that’s not so straightforward.
Greece can create euros under the cover of the Emergency Liquidity Assistance (ELA) program. And the Bank of Greece, through the Mint of Greece (IETA), can print euro banknotes.
The IETA need to have authorization from the European Central Bank to print Euros, but if the situation becomes chaotic, to avoid more chaos, Greece politicians can be tempted to do it illegally (or to do it with some sort of agreement from the European politicians).