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If you're in a bind, the Japanese "schoolgirl" outfit is a great pick. Even if you don't end up representing a character in particular, you'll definitely fit in with the crowd.

Find some pictures of your chosen anime character for reference. From there, you should be able to break down what you need for the costume. Some costumes are easier to put together than others. Many anime characters use a "schoolgirl" aesthetic; the clothes for costumes like this can be purchased at stores. More elaborate designs will require sewing on your part. If you want to have an attractive costume, it's important that your costume shows off your figure. This doesn't necessarily mean wearing a high-cut skirt, but it does mean you should do without costumes that blanket your figure.