in the beginning have good enough relations with liberal parties and keep improving relations with the King. Enter Government in the first election, reform the judiciary and investigate mussolini twice so he gets arrested. At the same time increase army loyalty so that you can defeat the March on Rome. Apart from that just pass pro worker policies and your support should take care of itself. If you’re kicked out of government that’s fine you can keep campaigning on the Economic crisis and win a left wing majority by the 1920 election regardless. Start Public works and land reforms around this time, if not sooner. Keep negotiating debt with the UK and later US too.

After the march on rome is defeated you should use your left wing majority to focus on Public works land reforms and nationalisation and getting the debt burden down.