When Alec and I went to the Wizarding World of Harry Potter last January, they had all the house robes, but they were over-priced and cheaply made. Instead I picked up an official Hogwarts crest patch so I could make my own custom robe.
I created the robe pattern by expanding a tent sloper to full length and adding bishop style sleeves and a hood. This is not a replica of the movie robes by any means. I didn't fully line this robe, it just has facing pieces to finish the opening. It is a generic Hogwarts robe, as it doesn't have any of the house colors for the lining. This way it can be any character's robe.
To make it, I was trying to use material I already had around the house. I didn't have the right kind of buttons in black, so I just found the right size in my button collection and covered them in the same fabric as the robe. I used a lightweight woven cotton that I had found at a fabric re-use place in Portland. I would recommend a heavier weight synthetic that resists wrinkling if I was going to make it again. There are in seam pockets on the sides for wands and chocolate frogs.
If you would like to make a robe similar to the one I made, here is the rough pattern. Print this out and blow it up until it is the size of the measurements stated. This is a very roomy robe that could fit a large range of sizes.
Here are the really rough instructions on how to put the pattern pieces together. If you've never sewn before this isn't enough information for you to put the robe together correctly, it's just a loose guide for people who've sewn from patterns before.
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