Advice and suggestions on how to teach pronunciation, in particular 'dark l'.
If they really want to pronounce dark 'l' ...
This is a difficult sound for many learners. If they can pronounce a clear 'l' (and of course this itself is difficult for speakers of some L1s) what they need to do additionally is to raise the back of the tongue towards the soft palate. The problem is that the back of the tongue is much less easy to control consciously than the tip, and it's hard to discover exactly what muscle movements are necessary to give the intended result. Here are a couple of things that might help:
Keep the front part of the tongue in contact with the teeth / alveolar ridge, as for clear 'l', and give the rest of the tongue a kind of gymnastic work-out, varying the shape of it randomly, feeling what different configurations are possible and noticing how the quality of the 'l' sound changes. Sooner or later, there's a pretty good chance that you'll hit on the right configuration for dark 'l'. You might be able to recognize it yourself, or you might need your teacher – or one of your classmates – to say "Yes, that's it!" Once you've located the right position and you know how it feels and what it sounds like, you should find it easier to get there next time. A supplementary exercise would then be to make a continuous long 'l' sound – as long as you can comfortably sustain - that fluctuates between clear and dark.
You might find that it helps to try to pronounce a long /u:/ vowel simultaneously, or at least to 'think /u:/' - this has the effect of raising the back of the tongue. Of course, there's then also the problem of using the two varieties of 'l' appropriately: clear 'l' before vowels and dark 'l' before consonants, before a pause and in words with syllabic 'l' such as 'table' or 'syllable'(!)
As with so many aspects of language, ability to recognize or even produce a difference in controlled activities such as minimal pair practice is no guarantee of ability to do the same in less controlled contexts where there are other – perhaps more important – things to pay attention to.
On the other hand .....Is it really so important? The distinction between clear and dark 'l' isn't phonemic in English (it is in Russian, for example) so it never serves to distinguish between pairs of words. Some native accents of English don't use dark 'l'. Many Welsh, Irish and South African accents use only clear 'l'. In other cases, the contact between tongue and alveolar ridge is lost, leaving only a back vowel articulation; this is traditionally associated with Cockney, but is now widespread as a feature of so-called 'Estuary English', and has lost the stigma it once had. This pronunciation is certainly also an option for learners.
Research indicates that lack of the clear/dark 'l' distinction in English used as a lingua franca for international communication doesn't restrict intelligibility. So .....
If your learners really do want to use a dark 'l', there are one or two tricks to help them. But it seems unlikely that not using dark 'l' has anything to do with their communication problems, and you and they might consider whether there aren't higher priorities than this on your short course.