I took his clothes he's supposed to wear under and made a pattern according to the upper part. You can see it as the smaller paper. I drew a pattern of the cloak - I made almost the same neckline, the P will be bell-shaped so I made the armhole diagonal to the original one, and a bit bigger.
This is the whole pattern - I had to join two pieces of paper with sellotape. I measured the desired total length of the P. I tried to make out geometrically the curve at the bottom but it was too shot at the front. I let it be (it will be comfortable to walk) and made the back pattern more circular. The back's also a bit wider.
I tried to make a pattern for the sleeve, but it didn't work..
By the way, I don't like that these types of cloak (with sleeves) are often forgotten and people wear only and only the basic cloaks with broach or band in the front. This one is called "karnáč" in Czech, but it's more known as gardecorps.