Proper handstand entrance

I am nowhere near a freestanding handstand in terms of the HS course (PE7 right now), but since we do practice entrance to a freestanding hs in the adult gymnastic classes I am attending, I'd like to know the proper way so to *try to) practice with good form.

From what I understood, you start standing, arms up, feet together. Then you lunge, the back leg gets totally straight in the lunge and lifts off (I think before the hands make contact with the ground). The front leg straightens only when the arms start pushing (or just a moment afterwards, not sure). Arms by the ears all the time.

They told us that the lunge should be pretty long - but that's vague. One instructor actually told us what seems to me a different technique: to put the hands on the ground even more forward than what you would get with just the lunge, kind of a little jump.

I am a bit confused, so any details about the good form would be greatly appreciated.

Googling around I found the attached picture, is this how it should look like - not that I am able to do that right now

The length of the lunge depends on your hamstring and hips flexor flexibility.

Thanks Alex, but is there an ideal length to aim for (given enough flexibility) or is it personal?

Regarding the arms, should they stay with shoulder completely open like in the picture I posted or is it acceptable to close a bit the angle when putting hands on the ground? I have seen videos with a gymnast maintaining the shoulder completely open until the body is parallel with the ground but then closing a bit the angle when putting the hands down (of course in holding the handstand she went back to completely open shoulders).