The Geometry of Fabric-Cutting for the Circular
First things first: Before you do any sewing, of any type, wash the cloth several times. Fabrics shrink the first few times they're washed, and better they should shrink before you sew them together than after. This is especially important in garments like cloaks, with vast expanses of fabric. Otherwise you are stuck taking your cloak to the dry cleaner every time it gets dirty, and that is expensive.
There are two basic types of cloak: rectangular and circular. But at the fabric store, there is only one kind of fabric: rectangular, of width from 39 to 60 inches, usually 45; and of any length we please. It is easy to get a rectangular cloak out of such material, and we won't go into it here. Instead, we will discuss ways to turn that rectangular cloth into a well-behaved portion of a circle.
When I speak of a "circular" cloak, I do not necessarily mean a full circle. Some very sumptuous cloaks, such as the ecclesiastical cope, are only a half-circle; and you can go on up from there to the three-quarter circle, and the full circle. (For some special effects, cloaks are made which use more than a full circle of cloth, but this is uncommon.) The more of the circle you use, the more folds you get (see fig. 1). The half-circular cope does not have very many folds; they are often richly embroidered, and almost all of that embroidery remains visible. The full-circle cloak hangs and swirls in many, many folds; and the richness of its effect is in the play of the fabric rather than in the surface. It is poor economy of effort to embroider a full-circle cloak, because half of the embroidery will be hidden in the folds.
A stiff, patterned fabric usually will look better as a half-circle; a rich, flowing fabric as a full-circle. The three-quarters circle is a versatile compromise which goes nicely with almost any fabric. Historical precedent should rule (if you're outfitting an archbishop, you must use a half-circle for his cope) but try for the best effect when there is a choice of fabric and style available.
If you have a wide fabric, the half-circular cloak is the easiest to make: simply cut a semicircle, hem and line it, and there-you-are. A good 60 inch fabric will make a cloak that falls 5 feet from the nape of the neck, which is usually enough - I am over six feet tall, and it still comes below the midpoint of my calf. The full-circle cloak is equally easy: just sew two pieces together, make a neck-hole, hem and line (fig. 2). But these cutting techniques are wasteful of cloth, the fabric you love may be 45" wide, and even 60" fabric won't give a ground-sweeping cloak for a six-footer. And thus, we come to the main portion of this discussion.