I often explain to students that if they misproduce prevocalic /r/ and substitute /w/ for /r/ as in west for rest that there can be confusion with  listeners.  I use the EWR 8 deck set prevocalic  and medial/final /rl/ deck to visually show the difference between rest as in your bed and west the direction.   I place the two cards side by side and visually show them the difference while working on auditory discrimination. Howay for Wodney...