There were two types of cloak in use; the sagum and the paenula. Both seem to have been some sort of brown or yellowish-brown color; apparently a particular type of wool was preferred, its natural oils making the fabric nearly waterproof. The cloak doubles as the soldier's bedroll on campaign.
The sagum is rectangular, blanket-sized or larger. It is simply pinned with a fibula at the shoulder or throat.
The paenula may have been semi-circular, circular, or oval. A circular or semi-circular pattern should be at least 45" in radius, and can be made from 2 or more pieces sewn together with a flat overlap seam. An oval should be about 60" wide by about 3 yards long. In any case, the paenula should be knee-length or longer. A semi-circle is pinned shut at the front; a circle or oval has a neckhole (approximately 6" diameter) and the front is slit open partway or all the way to it, and is also pinned shut. The paenula can have a hood about 12" square, sewn to the neckhole or to a semi-circular cutout in the straight edge of the semi-circular type.
About 3 yards of 60"-wide wool is needed for a semi-circular or oval paenula, and approximately 6 yards for a circular one. Allow an extra 1/3 yard for a hood, or piece it together from scraps. A semicircular paenula can often be cut from a blanket. (The large amount of fabric needed for a circular paenula seems to be an argument against its existence; it would be difficult to roll into a compact bundle unless the wool was quite thin.) A blanket will also make a decent sagum (remove any modern edging), or simply use 2-3 yards of wool. All cut edges should be finished with a blanket stitch or whip stitch.