A very good example of correct vocal placement in the classical voice is the singing of Franz Grundheber, a student of Margaret Harshaw. His voice is placed very forward. This is achieved through via a correct use of the /i/ vowel, which I wrote about in my last post.
The Old School taught students to 'place' the voice where they heard an /i/ vowel that had forward resonance. Every vowel was taken from a correctly produced /i/ resonance. Of course, it is helpful to have someone show you how to do this, but the autodidact can learn much from an example, even a Utube video. Of course, as with such things, you have to know something to know something. Nes pas?
The /i/ doesn't sound the same in every voice. Baritones don't sound like sopranos, and mezzos like tenors. But the canny voice teacher will know how to give the student feedback, and the even cannier student will be able to use their imitative ability to find the right quality.
Here is one hint that might help: the muscles of the outer ear (there are three of them) engage quite strongly on a well-placed /i/. In fact, the ears will feel pulled back and up.
Another hint: sing /i/ through /a/.