October

Today, I've decided I'll tell you how to make a Sabriel costume. Last year, I could hardly find anything aside from Dragon-Con people who were hand-embroidering silver keys on blue velvet. Sorry, I can't do that.
Here's what you need.
1-set blue robes (I got graduation robes from a thrift store)
1-black wig (Walgreen's)(Assuming you don't have black/dark brown hair)
2-belts (Preferably brown or black, if not, wrap the belt in brown/black tissue paper.)
1-bottle silver paint (I used acrylic)
1-sponge (clean, please)
Cardboard to put inside blue robes (so paints doesn't sink through both sides)
Scissors
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Those are the things you really need. From here on, you can vary. I'll tell you what I used, then what I think would be really cool.
My Stuff:
1-White-ish shirt
1-pair gray leggings (I have star spangled ones from Wet Seal.)
1-pair shoes, pref. high-tops (I didn't have boots, so I used high-tops)
Black tissue paper
7-popsicle sticks
7-dixie cups
Masking tape
1-small piece (4x6 ish) of cardboard
Green marker
Wrapping paper tube
Scotch tape
A few inches of sting or yarn
Facepaint or watercolors
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This is the "Plan B" idea
2-pairs black tights (get cheap ones)
1/2 yd-long lace trim
Safety pins, duct tape, or black thread and needle. Pick one.
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Abhorsen Robes
 1.  Put cardboard in blue robes.
 2. Cut your sponge in the shape of a key.
 3. Now use the sponge and silver paint to make the Abhorsen robes.
 4. After one side has dried, flip and do the other side. Don't forget the sleeves.
 5. This is personal preference,  but I don't think Sabriel could fight in long robes. I cut them off just below knee-length and make a slit most of the way up the thigh. I edged the raw edges in masking tape. It looked pretty neat, I think.

Armour
You can use your gray leggings and white shirt for the Abhorsen under-clothes, or you can go fancier.
 1. Cut a head hole and cut the feet off one pair of tights. LEAVE ONE PAIR ALONE!!
 2. Put the "shirt" (mutilated tights) and regular tights on. Don't wear anything underneath, except underwear you can get paint on.
 3. Paint scales all over the front of your outfit.
 4. Wait for it to dry, then put the outfit on backward, ie so the unpainted side is now on the front.
 5. Paint that side.
 6. After it's all dry (try to hang it up and let it air-dry over night) sew/tape/staple/hot-glue/pin the lace trim around the head hole and wrists.

Boots
Do this the day you wear your costume, after your armour is on. 
 1. Put on your high-tops (or other shoes)
 2. Cut two "bill" shaped pieces of black tissue paper to fit over the tops and sides of your shoes.
 3. Tape them on to your shoes.
 4. Cut a rectangle long enough to wrap around the upper outside part of your high-top. That's your ankle and lower calf. Cut it wide enough to go 1/3 to 1/2 of the way up your calf.
 5.  Tape it together with scotch tape. Tape it onto your shoe too, so it actually looks like a boot.

Bells and Sword
My bells were made the night before and I had no real materials. The sword was semi-decent.
1. Cut a hole in the bottom of each dixie cup.
2. Stick a popsicle stick through each hole and tape them in place.
3. Paint the whole thing silver. 
4. Put on a belt. (I used one of my dad's belts for extra length.) Buckle it on like a bandolier.
5. Tape the bells onto the belt, approximately where you want them. It won't be perfect.
6. Cut your 4x6 cardboard into a hilt shape. I painted mine gold and glued in a plastic emerald, but you can go how you like. If you want to be super-accurate, don't paint silver with a emerald. That's Nehima.
 7. Crumple the top of your wrapping paper tube a little, and tape the hilt and sword together.
 8. You can paint the sword silver with charter marks. I just did charter marks, since I was slapping it together, and it still looked fine.
 9. Tie the sword hilt onto your belt. (Not the bandolier.)

Other
 1. Don't forget your black wig, if you need it. My eyebrows were too light, so I got some face paint and darkened them.
 2. You can put on a thin wash of white face paint if you're tan or anything.
 3. Another accuracy detail. Get gray face paint (or watercolors are okay) and paint a circular charter mark on your forehead.
 4. I had to get a "burlesque" wig. That's one with a big Nancy Drew-like curl on top. I just got a thin gray scarf and tied it down. If you don't have that problem, don't worry.
  5. Have fun and make sure the person who announces you knows how to say "Sabriel" and "necromancer". Don't bother trying to explain who "Abhorsen" is. I had loads of fun and for the rest of the week people would look at me and say "Hey, you were that necromancer person right? What is a necromancer anyways?" :)

The Sabriel costume.

 The tissue paper boots are fun I think. They have a used and battered look. The blue robes used to be ankle length. You can't really see the sword, but if you look by the last of the bells.... Make sure the bells are firmly taped on. Mine had a tendency to slide out.

You could always go as Harriet Potter.

Star spangled leggings

Tips and Tricks 
1. Make sure your bells are firmly taped on.
2. Try wearing dark red lipstick. Not much, just enough so lighten your skin tone and make your lips visible.
3. If you are light-haired, wear black mascara. You might not notice, but your eyelashes are probably lighter than a dark-haired person's eyelashes.
4. Tell me how it went (give comments). Despite the fact no one knew what a necromancer was, I got first in my category at our school's costume contest.
5. Have fun and be confident. It won't work if you are shy or embarrassed. Just do it! :)