[Manuela]

I used to have a diagram made of cardboard on which I had drawn a section through the head, one could see the teeth, the part where the interior of the mouth should have been , I had cut out so that by wearing a red sock on the hand you could show the students exactly where to place the tip of their tongue. I hope my explanation is intelligible.
However lately I have found that it is easier if I just ask students to probe with the tip of their tongues inside their mouth and feel that if they go backwards from the upper teeth they will find that the "ceiling" of the mouth goes upwards abruptly, forming something like a cave inside the mouth. After they have acknowledged that it is easy to teach them the difference between Ts and tch for example. The first is pronounced with the tip of the tongue touching the upper teeth the latter with the tip of the tongue in the "cave"
I think that "r" and "l" follow the same ruling, "r" is pronounced with the tip of the tongue touching the teeth, "l " with the tip of the tongue in the cave. Now that I'm trying it out I start having doubts about the use of this for these two sounds but anyway I'm gonna post it.You may find some use in it.