This is my instrucctable of how to make a full hogwarts costume, from the hat to the wand. Im sorry but i dont have any pictures for this instructable right now butI might make an updated version.
Step 1: The Hat
Well first we are going to make the hat, although i guess you could do this in any order you really like. Ok, take a peice of bristle board and spiral it into the shape of a pointy hat until it fits the shape of your (or your models) head. once you have it perfectly fiting (make sure to have it a little big so it will stay on, thats where my mistake was)tape the crack with any kind of tape (except double sided!). take an old pair of black or dark blue pants (you could also use some material but pants are esier) that are sort of silky to the touch and cut off a pant leg. DO NOT DO THIS WITHOUT YOUR PARENTS PERMISSION! In the small end of the pant leg tie a not and then slip the cone of bristle board into it. trim the bottom of the hat so only about an inch of material is on the bottom of the cone. Tape the inch of material to the inside of the cone and your finished your hat! Dont worry if it's a little bit baggy it makes it look real.
Step 2: The Robe
Take a peice of black materialdown to your shins and wide enough to wrap yourself in once (you can make it bigger or smaller to your liking) and trim it to make it a little rectangular, the longer side the one that goes to your shins. grab a safty pin and pin it around your shoulders (over-lap the two sides of the top infront of your chest and while you hold it have someone pin it for you) You can make a house crest or hogwarts crest on paper or cardboard to make it look a little better, here are all the house crests: