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.