Combining Pitch and Larynx Height
Once you are comfortable manipulating your larynx and your pitch separately, you can try doing them at the same time. That means, while you are speaking in the female range, you also raise your larynx to reduce your vocal tract length. Start by following along with the exercises in this video.
It will probably sound pretty bad at first, and that is fine. Your goal at this stage is not to sound feminine, but to keep your pitch in your target feminine range, ideally between F3 and F4, and keep your larynx raised while talking (which you can feel by holding a finger lightly to your throat). If that feels strained while you are learning resonance, it is fine to work a little lower until the coordination becomes easier.
Your voice should sound more buzzy and brassy - a bright resonance (or bright timbre) - as opposed to the dark, hollow resonance of more masculine voices. That is a good thing! Watch this video to hear a great demonstration of this effect. You want your voice to be in the upper-right quadrant of the diagram.
Tongue Posture and Oral Resonance
On top of larynx height, you want to learn to arch your tongue up and push it forward to reduce the space in your mouth where sound resonates. To get the feeling, whisper "kee" (as in "key") and keep pushing the middle of your tongue up high for the "ee" - just below where it touches the roof of your mouth to make the "k" sound. Say it a few times while keeping your tongue clenched, pushing it a little higher each time. This is the smallest space you can make inside your mouth, the bright extreme of your oral resonance, opposite a yawn.
When feminizing other vowels, your tongue will be lower than for "ee" but still higher and more forward than in your masculine voice. You still want to feel a bit of that clench throughout. Essentially, you want to talk with a small space at the front of your mouth, which makes it sound like you have a smaller mouth than you actually do.
Gaining mastery over your tongue is one of the trickiest skills of voice feminization, but arguably one of the most important. Get started by practicing the exercises in this video. Then watch this video and try some sirens and trills across your range while raising your larynx and tongue.
Layered Practice and Whisper Sirens
Take your daily speaking practice and spend at least half that time talking with your larynx raised for a bright, buzzy sound. Then, as best you can, add in the tongue clench too, pushing it up and forward to brighten the sound even more. Experiment with your larynx raised or lowered, your tongue arched or relaxed, and your pitch high or low, as well as in a whisper.
Also keep practicing whisper sirens multiple times a day, but add a whispered "kee" at the end of each one to bring your tongue up. This lets you go even higher with the siren and make a really tiny dog sound. Hold those muscles in place at the top and really clench your tongue. At the same time, try to relax as much tension as you can in your jaw and neck while still holding the same shape.
Managing Tension and Recovery
This combination is likely to cause a lot of tension in your neck and throat at first, so do trills and yawn every so often to help the muscles relax again. You can even try lying on your back while practicing to force your body to relax.
If the jaw locks or your tongue roots hard, back off intensity and rebuild from smaller movements. Smooth control beats extreme posture. Sip water throughout your practice session and take a break when your voice gets too tired or hoarse.