This robe and the oversized t-shirt were made for me by a good friend of mine called Sue. She has far more ability in making clothing for costumes than I do and I'm thankful for her help, hence documenting her efforts here.

This instructable is going to be a little vague because I didn't take photos during the creation process. Hopefully, this information will be enough for anyone who wants to make their own version. The person this was made for (i.e. me) is six foot tall (183 cm) and about 180 lbs (about 82 kg). The cosutme is quite loose but you will want to alter some of the dimensions if the person you're creating is of a quite different size. These are rough plans and direct from Sue but if you're a little unsure on how things fit together, I'd recommend using paper cut outs to make sure the fit is good.

Step 1: Gather Materials

These are the base materials that were used:

Black cotton tablecloth, Medium (140cm x 230cm)
Black cotton tablecloth, Large (180cm x 270cm)
Thin, black material - I think I used polyester but muslin would also work
Velcro
Garden twine or thin wire

Tools that were used:

Sewing machine
Edge finisher
A previous costume made by the same person of a monk's habit for the initial shape
Suitable scissors
Paper and tape, if you want to make test pieces before cutting the real thing (advisable unless you're quite experienced with this type of thing)

Since table cloths were used, there was quite a bit of a hem already there and this was used for ease of creation of this project. The original hems were kept on the outside for neatness where possible. I chose 100% cotton so that it drapes well and be suitably ominous.

Step 2: Cut Out the Pieces for the Robe

There are six parts to this and you can see the pattern above. For each of the parts, use the existing hem or leave an extra 3 cm for creating a hem. There does seem to be a bit of a curve to the arm holes on the back and front pieces so you might need to alter as appropriate.

There are a few missing measurements which I'll list here:

Outer sleeve length - 67 cm
The distance around the neck including the overlap - 72 cm

Parts by the numbers

Right sleeve
Left sleeve
Back panel (entire width of the center piece in the drawing)
Right front panel (half the width of the back panel and a little extra for the overlap)
Left front panel (half the width of the back panel and a little extra for the overlap)
Veil - This is not on the plans but it is quite simple. It's made from the muslin like material, approximately 54 cm x 36 cm and one end trimmed into a semi-circle so that the material looks a bit like a tombstone.

Step 3: Cut Out the Pieces for the T-shirt

Cut out two pieces as above. The length (which isn't marked on the picture) was measured as 88 cm. Don't forget to add a little extra to the measurements to allow for the hem. The original hem is at the bottom edge, i.e. away from the neckline. A small amount of material was cut out of the neck section at the front so the material doesn't rub too much on the throat of the wearer.

Step 4: Sewing It Together

Each of the sections will overlap the material it's being sewn to by a few centimeters and the sewn overlap will be on the inside. You are likely to need to sew the fabric from the inside and then turn the material the right way round after you've sewn the relevant edges together. I'm wearing white clothes underneath to show what might be visible.

Sew the right sleeve along the angled edge to make a tube for an arm to pass through.
Repeat step 1 for the left sleeve.
Sew the right front panel to the back panel.
Sew the left front panel to the back panel.
Sew right sleeve onto the right arm hole of middle section.
Sew left sleeve onto the left arm hole.
For the hood, sew the fabric along one of the long edges to make a narrow, open ended tube. Fold the fabric in half and sew the other long edge to itself. The end with the thin tube will be next to your face and the just sewn seam will be at the back of your head.
Attach the hood with the previously short sides around the neck part of the robe. The back seam of the hood part should line up with middle of the back of the robe and the holes from the slim tube should line up with the top corners of the front panels.

The panels for the t-shirt are sewn to each other back to back at the shoulders and down each side. The t-shirt was then turn right side out to complete this part.

Step 5: Adding the Extra Bits

Five pieces of Velcro (each 5 cm x 2.5 cm) were sewn onto the edges of the front panels so that the robe can be closed. There's a gap of 7.5 cm between each piece of Velcro and the top piece is sewn just below the hood / panel join.

Thin Velcro strips (0.5 cm wide and long enough to go all around the inside of the hood piece) were sewn just inside the hood beyond the tube made from earlier (i.e. further inside the hood). The other side to the Velcro was attached to the thin material veil that was made in part 6 of "Step 2: Cut out the pieces for the robe" and around the curved part.

A piece of thin garden twine was doubled up and fed through the narrow tube in the hem at the front of the hood. This will give the hood some shape which will be useful later.

Using an overlocker, any spare edges of material were sewn so that they wouldn't unravel.

Step 6: Putting It All Together

The shirt is put on (over a dark, long sleeved top) and then the robe goes over the top of that. Using the thin Velcro, put the veil on and drape it down the front. Tuck the bottom of the veil behind the front panels. Using the Velcro, join the two front panels together so that the front panels cover the bottom of the veil.

The veil is made to be detachable so that the effect can be complete for some of the time. The rest of the time, you can remove the veil and cool down while keeping most of the costume on. The veil and the garden twine hide the face quite well but still keep a hooded character theme going on.

Step 7: Conclusion and Improvements

I did like how this came out and how quickly it was completed. The cotton drapes pretty and gives a good shape. I am happy with this result and would recommend this style for this type of costume. It can be the basis of several different costumes with only the need to change the accessories.

There are a few minor improvements that I might advise for people trying to make this costume:

Use as thin a material as you can for the veil. It gets pretty warm when I was wearing it and since there's no light emanating from your face, it could be thinner and still hide your face.
The sleeve shape feels odd to me - I suspect it's just a different type of clothing that I'm unused to. The sleeves feel a little too short and there's quite a lot of material that hangs under my arm.
The hood might need a little refining since it seems to fall forward sometime. Having said that, I have similar issues with other hooded coats so it is more likely to be my head than the design.
Pockets would've been a handy thing to have but there are alternatives that I used for this costume.