You know that awesome girl-version of Finn? If you want to cosplay as her, then follow these steps!
Buy the hat. You can't look like Fionna without her awesome hat! The hat basically looks like a white hat that wraps around her head, shows only the front of her face and has bunny ears. If you want to buy one, you can order it online. But if you choose to make the hat, it would be cheaper.
Get the backpack. It's a round, two-toned green backpack.
Buy the dark blue shirt. The shirt should have elbow-length sleeves.
Get the knee-length socks and black shoes.
When you are done with these steps, you can look like Fionna the awesome adventurer in the land of Aaa!
Ask your parents to buy this (unless you have your own money)! Ask them if you can dye your hair blonde if you don't have blonde hair, but if you don't want to dye it, you can buy a wig.