We as authors have been writing about people we aren't for forever. We find a way to empathize, we find a way in. Female characters are no different. All they are characters. They are people too. Instead of asking yourself, "How do I write this female soldier?" ask yourself, "How do I write this soldier? Where is she from, how was she raised, does she have a sense of humor? Is she big and tall, is she short and petite? How does her size affect her ability to fight? What is her favorite weapon, her least favorite? Why? Is she more logical than emotional? The other way around? Was she an only child and spoiled, was she the eldest of six siblings and a surrogate mother? How does that upbringing affect how she interacts with her team? Etc etc. and so forth." Notice how the first question gets you some kind of broad, generalized answer, likely resulting in a stereotype, and how the second version asks lots and lots of smaller questions with the goal of creating someone well-rounded. One would hope, really, that we as authors ask such detailed questions of all our characters, regardless of gender. So let me, at long last, actually answer the original question:"How do I write a female character?" Write her the way you would write any other character. Give her dimension, give her strength, but please also don't forget to give her weaknesses (for a totally strong nothing can beat her kind of girl is not a person, she's again a type - the polar opposite yet exactly the same as the damsel in distress). Create a person.