I’ve learned a couple of things from voice coaches over the years (they were invited to our group meetings, I didn’t take individual lessons).
First, I listen to the pitch of my voice. I get that in my head, and then go up half an octave. In other words, from the pitch, I sing do-re-mi-fa-sol. I talk at sol instead of at do. This gives me a higher pitch, doesn’t sound falsetto, and is not so high that I couldn’t modulate my voice around this note.
Second, I open my throat when I talk. Similar to what they tell singers, not to sing from the throat. This is a subtle change, but it eliminates the male buzz in the throat, Remembering to do this is probably the hardest.
These two will not change your voice because there is still the issue of resonance. If a male and female sang at the same pitch, you could still tell me which voice is which because the male’s voice sounds bigger. He has a bigger space for the sound to vibrate in his head.
Some people compensate for this by talking in a whisper or being breathy. The problem with these techniques is that you will lose any feminine quality as soon as you have to put volume into it.
Instead, what I learned is to make use of the smaller space in your head, namely your nasal cavity. It requires experimentation because you don’t want to sound exceptionally nasal when you talk, but I find pushing somewhere between half and two thirds of the voice into your nose will make it smaller and more feminine, yet not incredibly nasal sounding.
I find these techniques work well enough for making “small talk,” quick conversations with people.
When you get comfortable with this, you can go add to it. A little more advanced would be to make your vowels longer and your words flow smoothly (legato) whereas men talk choppy (staccato). Also learn to talk with your hands and modulate your voice a little to emphasize words as you speak. These are secondary techniques that aren’t incredibly necessary to start, but will help once you get the first few steps down pat.
There are more advanced things too (word choices or talking around a subject and going off on tangents mid thought) that I’m not going to go into.