Grahame: here is how I did it on my Mac/OS 10 El Capitan:
System Preferences > Language and Region > Keyboard Preferences. Click the + key, click English to bring up various english keyboards on the right, highlight US International PC keyboard and click Add. Close out of System Preferences. Now in the menu bar at the top you should see a US flag. Clicking on that will allow you to switch to the US international keyboard.
I can type accented vowels by first typing ' then the vowel, the ñ by holding down OPTION and typing n, then retyping n, ¡ by holding OPTION and typing !, ¿ by holding OPTION, SHIFT, and typing ?.
I don't know how to do this on a PC but it should be similar. I also don't know if there are characters that you use in Italian that we do not in Spanish so you may have to experiment. But this should point you in the right direction.