There is a definite knack to cutting the angles. 36 years ago as a newlywed, my dad helped me cut the cornice for a floor to ceiling cupboard that I made for our new home. I only needed at most 1/3rd of a length of cornice but had to buy the whole length - painful on an extremely tight budget in those days. Used the mitre box but kept getting the 45 degree angle in the wrong plane so had to remeasure and cut several times. At last we got the “angle of the dangle” correct, only to realise we’d made an internal 90 degree instead of the external one needed. Turned out the whole length was only just enough to complete the job. We often laughed about that. These days I’m sure YouTube or my mate Google has instructions.