A fun and effective approach to teaching the "er" sound (works for /r/-blends too!):
These cards are designed to give multiple cues for articulator placement and sound production of the “er” sound. We’ve found these to be highly effective – and fun – for kiddos struggling with this tricky sound. We incorporate a picture cue (e.g., growling tiger for the “ger” sound) and a gestural cue (e.g., “growling mouth”) to help children remember proper production. After you introduce the picture cards you can use the gestural cue as a non-verbal reminder for placement of lips and proper sound production.