Monster High characters wear a lot of individual items, so my first instinct is to list all of her clothes and accessories. She wears red jeans, a striped off the shoulder tee, and a camisole. You may be able to find these items, and simply add the details with fabric paint. She also wears knee high sneakers, a belt, white glasses, green headband, and fishnet arm warmers. If you can make or find these, she also wears earrings and a necklace.
Arda has a mixed blue wig that would work for her (Candy Striper in Blue Danube). It's not perfect, but takes the work out of adding wefts to a base wig (option 2). She wears matching red lipstick and nail polish, with maroon eye shadow. Her skin is pale grey.
Tackle each item as you can. I would get the main items done, and if you have time, gather/make the smaller details.
If you need any help, please visit the Monster High thread in the cartoons forum. Good luck!