Sing along to female singers, and try to speak like female characters in shows and movies. Try to imitate their tone and inflection as best as you can. Record yourself doing it. Try different things... change different muscles each time, try to make the tone come from different places, use different throat shapes, tighten different muscles, and listen for which ones make you sound more feminine. Once you get a good idea of which muscles and which tonal area is the right one, try to incorporate that into your own speaking tone, inflection, and resonance. Practice EVERY day. 15 minutes is fine, as long as it's every single day. Because it will take months and months and months of repetition and muscle memory to become natural.
That's what I've been (attempting) to do at least. Singing along to Karen Carpenter, Joan Baez, Kate Wolf, and Joni Mitchell. Plus some occasional help from Andrea James' course, Kathe Perez's videos, and a lot of Youtube videos on the topic just to point me in the right direction. Really, it's just about practice though.
Also, try holding books on the sides of your head, just in front of your ears. This will block the internal noise feedback that makes your voice sound different to you than it does to everyone else, and you'll hear what everyone else hears. Then you can speak in real-time and see how you really sound to others, so it's easier to practice.