Confusion about how to pronounce the "dark L" consonant sound

I watched the first Youtube video you referenced, done by Rachel, and I think several of the observations are good, but overall, it misses the point. The important difference between dark l and neutral l in English is retraction of the tongue body for the dark l. (I'm calling the non-dark l "neutral", because I don't think l in English is truly light -- just sometimes not dark.)

The position of the tongue tip doesn't matter. When the body of the tongue is pulled back, that may affect the position of the tongue tip, but that's just because the tongue tip and the tongue body are connected -- they're both part of the tongue. It's the retraction of the tongue body that makes the l dark, whether or not there is any difference in the position of the tip.

Rachel talks about the pronunciation of "real" and observes, correctly, that there is an extra vowel sound that intrudes between the [i] and the [l]. But that is not what makes the [l] dark. When the tongue body is pulled back for the dark l, the body of the tongue has to move from front to back, since it was pushed to the front to make the [i] vowel, and it has to be pulled back for the dark l. To get from one place to the other, the tongue body has to move through an intermediate central position, and this is what Rachel hears as an extra vowel sound between the [i] and [l]. This central glide is predictable if you have a correct idea about how dark l is articulated.

The rule for dark /l/ is that we always use dark /l/ when /l/ isn't followed by a vowel. So in the word falafel the first /l/ is clear, the second is dark.

Arguably, there are two parts to an /l/ sound. The first is caused by the redirection of the air as it travels out of the vocal tract. This is caused by the tongue tip making firm contact with the alveolar ridge, the flat shelf behind the teeth. This redirects the air sideways out of the mouth giving a special kind of resonance.

The second part of the dark /l/ sound, concerns what kind of vowel quality the sound has. The quality of dark /l/ is something like cardinal vowel 7, a high mid, back, rounded vowel. For learners, you could aim for an /ʊ/ sound, which would do fine. For a vowel like this, the back of the tongue is raised, but it's not easy to consciously raise this part of the tongue: it is much simpler to just aim for an /ʊ/ vowel. This is the vowel we use in the word put.

• To make a good dark /l/ then, make an /ʊ/ sound, whilst putting the tongue tip on the alveolar ridge behind the teeth.

This effectively will achieve what is described in the fourth explanation.

Edit note: The vowel behind dark /l/ differs between speakers of RP. Alan Cruttenden in Gimson's Pronunciation of English describes it thus:

Variations in the quality of the back vowel resonance associated with [ɫ] are found among RP speakers, with a range extending from [ö], [ʊ], or [ɤ] to [ɔ:] or [ʌ].

7th Ed, p. 216

Some readers might find that using /ʊ/ sounds a bit too Londony (see David Garner's comment below). However, it's probably the most commonly found, and is easy for non-native speakers, who will recognise the quality from the FOOT vowel, in words like put.

Dark L --> Why not simply add voice to /h/ ? f-v-f-v s-z-s-z h-ɫ-h-ɫ

Works for me. I'm mid-atlantic being born in Ontario and raised in South England. I use a dark L in milk but a clear L on the end of many words like kill. I love the accents from elsewhere in the UK so I'm not recommending mine, except to say I'm usually understood well enough.

Phonetically, the dark l [ɫ] is basically the light l [l] velarized.