A quick fix for last-minute costume needs... or those of us in climates unsuitable for heavy fabric or sticky polyester robes. I made this for a Potter convention this summer in Florida, but without decorations it could be used for many purposes.
I used pieces of five tshirts, but it could be done with four depending on the sizes; a medium made a large hood, and I used larges for the body pieces and another for the arms.
My apologies for the lack of photos, I might make another for my friend, if I do I shall add some :)
Lay out your tshirts and organize; you'll need the medium for the hood, one large for the sleeves, and the other two or three larges for the body of the robe. Feel free to get creative with scraps if you have big pieces lying around in the color you want.
Take the medium and cut it off at the bottom of the sleeves, so you have a wide tube of fabric. Cut through one side. Fold it the other way along the original fold (so it's inside out) and with the fold side at the top, pin the unhemmed side. Curve the line of the pins at the top (otherwise you'll end up with a pointed hood).
Take one of the larges, try it on to make sure it's loose enough, and then lay it out flat. In the front, cut strait up from the hem to the center of the collar. You can remove the collar if desired, but I kept it on.
Lay out the large shirt (or your scraps) and cut out sleeves. You could cut the sleeves off the top of the robe and make them all of one piece of fabric, but I didn't have an extra shirt, so I left them on and added length to them. If necessary, make a pattern first and wrap it around your arm to make sure you like it. Mine ended up being a very slight trapezoid shape, to make wide bell-sleeves.
Once you've cut the sleeves, fold them over (right-side in) and pin.
Lay out your other shirt (or two, depending on how long you want your robe to be) and cut across below the sleeves. Cut open the tube and lay one edge against the bottom of the mostly-intact tshirt (right-sides in). Pin together.
Now it's time to start sewing!
With a machine, this is a snap, and only takes a few minutes for each piece. Start with the hood and sleeves, sewing along the pins (make sure to reverse on the ends!).
Turn these right-side out when you're done.
Pin the hood (outside to outside) to the collar of the intact shirt. This might take some creative stretching and finangling, and you'll probably need to sew the sides of the hood to the front of the shirt (along the cut edges) for a couple inches. Since mine was an experiment, it doesn't look as clean, because I did it in separate sections, so it isn't a smooth curve on the inside seam.
Now, take the sleeves and turn them back so they're inside out again. On end will be wider than the other, and the narrow end is the one you'll be attaching to the short sleeves of the main shirt. Slide the wide part of the sleeve down the short sleeve until you can line up the other ends. Pin the ends together (one sleeve might be slightly bigger or smaller than the other, just gather a bit on the seam or in a spot that isn't as obvious and continue sewing). Repeat for the other sleeve.
Sew the pieces of the main body together where you pinned them. Then fold everything back the right way and check the length of the sleeves and bottom hem. You can hem the sleeves if they're too long, or just cut off the excess and leave it.
If you want to add some kind of closure, go ahead and do it now. A button and a loop of ribbon, or some kind of clasp, a frog or a hook-and-eye... whatever you want.
Now you can add decorations: add strips of a contrasting color shirt around the edges of the robe, or use bias tape, sew a patch on, put in a wand pocket, add lace trim, whatever you like...