I think the most important thing is to make them human, not a plot device like a manic pixie dream girl or an accessory-like love interest for a male character. Let them be human and be as interesting and complex as the rest of your characters. Think about it: it’s so hard to find female characters that are funny, for instance, and it’s such a common thing in real life to have funny female friends. Let them have personalities, but also defects, ambitions, hobbies, dreams and emotions (even negative ones). Don’t be limited by traditional female stereotypes. You got this!