Operating human vocal hardware is a task we perform instinctively as men and women. Altering the way we speak takes knowledgeable instruction and continual practice. Attempting to change voice patterns to replicate the opposite sex can be explained, but in the end, you must pick and choose the specific tips that best fit your voice.
Find a wall-mounted mirror in which you can clearly view your neck and mouth.
Place the thumb, index finger and middle finger on the larynx (where your Adam's apple protrudes). The thumb should be on one side; the index and middle fingers on the other side. Speak a phrase or sentence in your normal voice. Pay attention to the slight, upward movements of your larynx.
Gently push your Larynx upward toward your chin without pushing inward in a harsh manner. Speak your phrase again and notice the restriction this pressure places on your vocal chords' resonation. Your voice will be at a higher pitch.
Block off the air flow to your nasal passage; use your mouth to breathe. Hold your nose closed with your free hand if needed. Repeat Step 3, and notice your voice is at an even higher pitch.
Perform the same action while attempting to make your voice as breathy (forcing extra air out with your sound) as possible.
Continue speaking this way until you feel you can perform the action without manually raising your larynx and/or holding your nose. Continue to lift your larynx as high as possible, limit the use of your nasal passage and force air out with your voice.
Don't overdue it. Practicing for a few minutes a couple times a day is enough to begin. You are forcing your vocal instruments to constrict and resonate less, which can be damaging if overdone. Over doing it will also make it more difficult to achieve the results you are looking for.
Ask a woman if you can feel her larynx while she speaks to get an even better idea of how a woman's vocal instruments operate.
The vocal instruments are incredibly delicate. Handle them with care.