I used the above template for my headpiece—you can click through to make the image larger if needed.
this template was created & posted by Livejournal user eve11 in 2008 (!!!), as a headpiece tutorial for the Doctor Who characters The Sisters of Plenitude (cat lady nuns!!!).
(I don't watch Doctor Who myself, but I appreciate Eve for drawing up & sharing such a useful template!)
while not exactly the same, The Sisters of Plenitude wear the kind of large, 'horned' headpiece that I was looking for—apparently inspired by the cornette headpieces traditionally worn by an actual order of nuns, The Daughters of Charity.
Eve recommends using white buckram fabric for your headpiece. because I was just using my headpiece for a Halloween costume, however, I just used a large sheet of construction paper for mine ¯\_(ツ)_/¯
the construction paper folded easily while still being able to retain its shape. I didn't attempt to fold the 'wings' back on mine, though, so my headpiece ended up looking more like The Flying Nun, lol. 🤭
Eve has provided step-by-step instructions to building the headpiece in the archived Livejournal post above. if you would prefer a video tutorial to follow while making this project, Yeoman Locksly has a three part tutorial on Youtube for this template as well.
good luck, meow-meow! ᓚᘏᗢ