There are a number of styles of cloak. The simplest is a half-circle or full-circle with a hood. Here's a simple pattern for a half-circle cloak:
Peasants cloaks are functional objects. They are generally long, reaching to the mid-calf or even lower, and are made out of sturdy fabric. If this is to be your rain cloak, make it out of wool (see Survival)!
Nobles cloaks can also be functional, but made of finer fabrics. Use a nicer wool, or sandwich a waterproof layer between two "appropriate" layers. This will be hot, but you'll stay dry!
Dress capes for Gentlemen are another matter. They are shorter, for one thing, ranging in length from wiast to mid-thigh. They are often elaborately decorated and trimmed. Typically, a gentleman will wear the cape over one shoulder (for maximum swash-and-buckle), although it is possible to wear it as a cloak, if His Lordship is cold.