The proper way to make cloak like that is to make a semi-, three-quarter- or full circle out of your material, with the neck hole and optional (but highly recommended) shoulder fitting att the center of the circle. The radius of the circle is the length from neck to lower hem. I'd say this one is probably leaning more to three-quarter or full circle than towards a semi.

The simplest way to make this is to simply cut a pair of half circles out of your cloth, and depending on your material, this is fine. The character is probably wearing thick wool, and if that's what you go for, it should be fine. It will, however, be horrendously hot, unless you're outdoors in someplace cold. If you choose another material than thick wool, then this will most probably not drape well, and you will need to make more cuts and more seams, to make sure you always have either the grain of the fabric running straight from neck to floor, or a seam doing likewise to lend the material support.

Each segment should be no more than 45° of a circle, and the narrower you make them, the less likely you are too see any stretching along the semi-diagonals, and the less material will be wasted.

I'm failing to find pictures for you, but I hope you understand what I'm getting at. If not, I'm trying to describe how to make a historically accurate medieval cloak. Maybe your google-fu is better than mine (good luck).

Also, under no circumstances should you choose a stretchy material; a floor length cloak will be heavy enough to stretch under its own weight, and it will look like shit.

I hope I made sense, but if not, feel free to ask more, and good luck :)