I'm the furthest thing from an expert, but from what I've found by dinking around a bit, one of the most crucial aspects of a fem voice is where you resonate from. Try making a "Mmmmmmm" or "Nnnnnnnnn" sound for a while. It should feel like it's resonating near the tip of your nose and lips. Try to speak from there. I've got a practice pdf here that I think I yanked off of an old reddit thread or something.