The problem with recourse to specific phonetic descriptions is that both English /r/ and /l/ have a lot of 'positional variants', variations depending upon what word they appear in, where they are in a spoken syllable, etc. Also students typically do not transfer static one-dimensional diagrams into the 3-D dynamic movements required to produce speech.
I advocate a systematic approach that requires no technical terminology, no phonetics on the part of the teacher or the students. It does take time and so you need to work at the two sounds over a set of classes.
See:
ELT in Japan (ELT-J): Teaching English /r/ and /l/ to EFL learners: a lexical approach (parts 1-3 final)
Charles Jannuzi
Fukui, Japan