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:
Step 3: The Wand
For the wand you have to use a lot of creativity. go find some fresh wood (don't kill a tree for no reason so don't cut one down, if anything only cut off a little bit) that is about the length of your forearm measure how long you think the handle should be (a little bit longer than your hand grasped on one end) and make a mark with a knife or a pen (let your parents help you with the knife). Make sure you chose the thicker end of the stick for your handle. Carefully remove the bark trying not to alter the smooth round shape of the wood within. DO NOT remove the bark where the handle is! Then you can personalize it! Add some special markings or your name, or even change the shape of your wand by getting your parents to help you carve the wand to your liking. another trickyer way to make the handle is to remove ALL the bark and make the non-handle part thinner by carving it with a knife. this way you can make the handle nicer by shaping it differantly, but i dont recomend this way to anyone who hasn't made a few regular wands first. After you've finished making the wand you can stain it (with the paint-type stain, not grape juice) and this will make it look really amazing.