I drew out cloak patterns and scanned them into the computer a little while ago. I need to find them and then I'll post them.
Basically the Birrus Britannicus is one length of wide fabric (join two lengths if it's not wide enough ) It is longer at the back to carry rain away past your thighs and just long enough at the front not to soak your knees.
The corners are rounded off so that they don't trail. The neckline on mine is an oval, slightly deeper at the front.
The hood is deep enough to come right round to within two inches of the front facings so that it's warm at my throat even when the leaf brooch I use is fastened through the two layers.
A more authentic fastening would be a ring headed pin or a carved bone, wood or antler pin with a hole and a cord. The pin goes through the two layers and back out again and then the cord is wrapped figure of eight style to keep it secure.
Try to cut the hood so that there is no seam over the top of your head. This way there are no shoulder or head seams to let rain through and no shoulder seams to rub under a pack.
To free your arms entirely both fronts are thrown backwards over your shoulders.........think Roman squaddie or knight in armour style
If you wish to belt it into a tunic bring the back edges forward, fasten a belt loosely over the top and then raise your arms pulling fabric comfortably upwards through the belt. Then fasten it a little tighter to keep things there.
cheers,
Toddy