Many students have trouble making the l/r sounds and the key is in what muscles and parts of their mouth they are using. Here is a brief explanation you can tell to your students.
To make the L sound, usually your tongue touches the back of the top row of teeth. The front of the tongue comes up, but the back of the tongue remains relaxed and doesn’t touch the back teeth. Also your throat remains open. Your mouth and lips should be relaxed and in an open ‘eh’ position. Your lips shouldn’t close at all. To release the sound your tongue moves away from the back teeth. When I make this sound my tongue pulls directly back. However, to exaggerate the sound, you can flick your tongue down, like when you say ‘la la la’.
To make the r sound, your tongue doesn’t go up or down. The front of the tongue is still, but a little raised in the middle of your mouth. However, the back of your tongue flattens down a little bit. You can almost feel it touching your back teeth. Your throat doesn’t close. Your tongue doesn’t block the back of the throat. Your jaw moves down a little and then back up as your lips close slightly to make the ‘ahh—eh—r” combo sound. In the end your lips end up looking like this:
When your students practice making the sound alone have them use a mirror. They can watch you and then watch themselves to try to fully imitate the shape of your mouth. Also, after they have a general idea of how to make the sound, always have them practice the sound in context by saying a full word. Just practicing the sound alone, really only helps them make the sound alone.