There are plenty of excellent commercial patterns for cloaks
out there, so for now we won't bore you by listing them. Instead,
here are a few of the online references for cloak shapes that
will help you make your own or even just decide on a pattern.
Shape Index and guides
SCA
Cloak shapes: covers all three basic cloak shapes - rectangular,
circular, and shaped
Circle
cloaks: covers 1/2, 3/4, and full-circle mantles and capes
(like the girls' Narnian camp cloaks)
Butterick
4320: Yes it's a commercial pattern, but even just looking
at the flat shape of cape D will give you an idea of how to
make the Pevensies' Coronation
capes
Cape
Instructions
Philo's Quick Cape: Tutorial
for a quick and easily-adaptable cape, drafted by Philo. Pleated-neck
rectangular with a mandarin collar.
