The question is what is a mixed voice?
Form a standing wave with the vocal tract’s resonating tube-cavity. The result is that most of the amplification is through resonance and standing wave. If you form the standing and resonating wave correctly, it will automatically have the mixed or full sounds—because the entire vocal tract is resonating.
To “mix” this sound, it is more a matter of subtraction—put a barrier to muffle or absorb the head or chest voice.
The question now is—how does one form the standing wave resonating tube cavity to begin with? Obviously, if your posture isn’t correct, it won’t form correctly. And if you have back of mouth obstructions it may not form the highs correctly. So, the first step is to get your posture correct and then your vocal structure correct, which is what is described extensively in Vocalposture.com.
From a “ZenSinging” perspective, the question should be, why don’t you already have a mixed voice? The Zen answer is that you lost it. So, the focus should be on restoring what was lost, instead of getting the mixed voice. The restoration techniques are different from standard singing training techniques.