How to Create a Hermione Granger Costume
To create an easy Hermione Granger costume, find a basic white button down shirt, and tuck the shirt into a black or gray pleated skirt that reaches your knees. Then, put on a v-neck sweater in black or gray over top of your shirt, as well as a scarlet and gold tie. Wear a pair of knee-high gray or black socks with simple black shoes, like Mary Janes or loafers. To complete the outfit, throw on a long, black robe with a hood, and blow dry your hair so it's wavy and somewhat bushy.
Wear a white shirt. Find a basic white button down shirt. Find a skirt. Look for a gray or black skirt that almost reaches your knees. Pleats are optional. Choose a vest and tie. Pick a V-neck vest in gray or black to wear over your white shirt. Select a tie in Gryffindor colors (scarlet and gold) to wear underneath the vest. You could also wear a button down cardigan instead of a vest, especially if you're not going to be wearing a robe. Select socks and shoes. Look for knee-high socks in a solid color, such as gray or black. Wear black or brown shoes in a simple style, such as Mary Janes or loafers. Wear a robe. You can find one at academic bookstores, costume stores, or charity and thrift stores. Just be sure to find one in nice condition, as Hermione wouldn't have had ragged or worn robes. Ask any lawyers or academics in your family if they have a robe you could borrow. If so, be careful not to permanently attach a Hogwarts insignia to the robe. Decide which Hermione hairstyle you want. If you're portraying a younger Hermione, go for a bushy, somewhat large hairstyle. If you're depicting an older Hermione, go for loose waves that may be pinned against the side of your head. Remember, Hermione has medium-brown hair, so avoid light brown or blonde hair colors. Style your hair. If you're portraying a younger Hermione, blow dry and brush your hair to create lots of volume. Don't use hairspray, but let your hair be frizzy and loose. To easily create wavy hair, rub mousse through your wet hair, part your hair into two sections and braid each part. You should end up with two braids running down the sides of your head. Let your hair dry in braids before unbraiding. Your hair should now have kinks and waves. Keep the makeup minimal. Hermione isn't known for stylish dressing and lots of makeup. Instead, keep your look natural. Consider applying concealer, a basic powder, and a bit of blush. Use a pale pink lip gloss for a younger Hermione or a matte lipstick for an older Hermione. Apply eye makeup, optional. If you're portraying an older Hermione and want to wear eye makeup, use natural shades, such as taupes or beiges and avoid eyeliner. Use a basic mascara and keep the eye makeup simple. Avoid any eye makeup if you're going for young Hermione, but consider filling in your eyebrows to make them bushier. Carry books. You can simply carry a few around or use a book bag. Be sure to carry magical books. To do so, choose several books and make covers for them out of brown butcher paper. Then decorate the covers and title them appropriately. For example, these could be textbooks like, "Muggle Studies," "Divination," or "Potions." To use a book bag, stuff the bag almost full with book-sized objects. Real books would be too heavy. Try using small, empty cardboard boxes or square chunks of Styrofoam underneath a few real books that you've decorated. Wear a time turner. Find a tiny hourglass and place it on a gold chain around your neck. If you can, find one with rings that encircle the hourglass. This will be the Time-Turner that Hermione used to get to all her classes in "The Prisoner of Azkaban." Attach a Hogwarts badge to your robe. Make or buy a Hogwarts badge that shows the four houses. You could also make a silver S.P.E.W. badge like Hermione wears to support the Society for Promotion of Elfish Welfare. Simply use silver-colored paper or stretch Aluminum foil over cardboard. Write S.P.E.W. in decorative lettering. Carry a wand. You can purchase a wooden wand or craft your own. If you plan to make your own, decide how realistic you'd like it to be. You can make a wand out of rolled up decorative paper, or carve your own. Whatever you choose, make her wand a little less than a foot long. Consider adding embellishments, such as vines, along the end of the wand. Make it reflect Hermione's character. 
Wear a white shirt. Find a skirt. Choose a vest and tie. Select socks and shoes. Wear a robe. Decide which Hermione hairstyle you want. Style your hair. Keep the makeup minimal. Apply eye makeup, optional. Carry books. Wear a time turner. Attach a Hogwarts badge to your robe. Carry a wand. 