Let's get something out of the way from the beginning: I am not an expert when it comes to women. Stay with me for a moment, it's actually a good thing. It means I spend less time focusing on a concept or idea and more time writing about the character as a person. With that in mind, I want to make sure to set the record straight that I do, in fact, know a few women: I am the son of one, have represented several in various legal dilemmas, am married to one and am currently raising two. My point is that these are all people that I've gotten to know and/or who are a part of my life. As a male writer, I believe that is the key when writing about women characters: you cannot lose sight of the fact that you are writing about a person. More importantly, all writers should want a character to resonate with other people, not just with one gender or the other. Again, focusing on the character as a person is what's critical. Otherwise, you may come across as reinforcing certain stereotypes and alienate your readers.
Next, you'll have to decide what are the strengths that define this woman character who you've created. Very few of us know a single human being that embodies all of the "strong" qualities that we like to see in leading literary characters. Usually these heroes or heroines have a number of qualities that strike positive chords with the reader. In real life, the friends and family members who we look up to might only embody one or two attributes that would brand them as being a "strong" person. Like all of us, even a strong leading lady in your cast should have some sort of flaw, quirk or shortcoming. Real people aren't perfect, and the ones that seem that way are just very good at hiding skeletons in the closet. Your characters shouldn't be perfect either. If they are, create a secret they are afraid that everyone will find out. Now, you've humanized your super hero and given the reader something to invest in (i.e. watching the flaw get overcome or damage control when the secret is discovered).
By way of example, in "Of Murder and Monsters," Savannah Keller definitely is hiding a secret and the lawyer defending her, Sean Valdez, tries to uncover the mystery. As the story unfolds, it becomes questionable as to whether Savannah's reluctance to divulge her secret is as much to protect Sean as it is herself. Is she doing this for selfish or selfless reasons? That's for the reader to decide and, in doing so, determine in what kind of light do they see Savannah.
What I'm getting at is that you don't have to be an expert to create a strong woman character, you just have to be an observer of your own life and the women in it. You can pick and choose from the myriad of qualities from the women you know.
What kinds of qualities do you feel embody a strong woman character?