Do I need to write down all of my working out?

Yes, in order to maximise the amount of marks you obtain you must write down all of your working as often the answer is only worth 1 mark whereas the working out makes up the rest of the marks. Always show your working out in a clear, methodical and coherent manner so that the marker has no trouble in understanding how you came to your answer. 

If if you’ve shown your working out but your answer seems wrong, keep all of it written down and go back to it later - it might be right and even if it’s not some of your working out might be.