Wear minimal (or no) makeup. Hermione doesn't wear that much makeup She might wear a little blush and a skin-color eyeshadow, but nothing too fancy. For movies, Hermione likely wears makeup. If Hermione was a real live human being, she probably wouldn't wear any makeup. You can wear a little makeup or no makeup at all. For makeup, try some light blush, brown or plain mascara, some concealer, if you need it, and maybe foundation. This will highlight your features naturally, giving you natural and simple beauty.
Wear comfortable clothes that also look neat and presentable. In the Deathly Hallows, Hermione was wearing a white collared shirt under a striped blue and black long sleeve shirt. On top she wore a navy blue sweater with it unbuttoned at the top. This type of clothing shows you know the rules and dress code. Try layering things like a button-down collared shirt underneath a sweater, or a simple white blouse with a blush-coloured cardigan. Simple Polo shirts are also very comfortable and flexible, while still looking put-together and studious, and Hermione would choose something like that. Try to show people that you know the dress code, stay comfortable and look smart.
Don't really worry about your hair or clothes too much, as long as you look together. Don't dress inappropriately, if you wear crop tops, too much makeup and very low-cut dresses you will not be taken as seriously as someone dressed in a more toned-down fashion. People may consider you superficial. To avoid this, dress casually and neatly. Have good hygiene and you should at least look presentable. Although books are more important to Hermione then her looks, she doesn't let herself look dishevelled. Looking orderly and together helps convey your smarts and organization to teachers. Hermione cares more about her brains than her appearance. This was something she declared once. Hermione was described as a girl with lots of bushy brown hair. Try this: don't dry your hair after a shower; instead, let it air dry. This will leave it flowy and bouncy. If you have thick hair, try to be careful with this step.
Wear a S.P.E.W badge. If you want to be like Hermione, you can try wearing a S.P.E.W badge or a Time Turner necklace.