How did Nero kill Paul?

The most reliable evidence we have regarding whether Nero killed Paul is from Clement of Rome, writing around 95 CE (1 Clement). He spoke in general terms of Paul but seems to have been unaware that he died in Rome just a few years earlier, believing that he ended his days in the 'west' (Spain).

In fact, we do not know how or even when Paul died, although a second-century Christian tradition claims that he was martyred in Rome by Emperor Nero in the mid-60s of the first century.