The youngster should put the tip of the tongue on the upper gum behind the central incisors to produce the /l/ sound. The child should take a deep breath and use voice to make the /l/ sound. With the voice going, gently push the underside of the tongue into palatal arch with the tip of a tongue depressor. With the lateral segments of the tongue contacting the upper molars and tongue blade in the palatal arch, you will hear a perfect /r/ sound.
Do this several times and have the patient kinesthetically feel the tongue position in relation to the intra-oral position. Finally, have the patient open the mouth wide, comfortably, and say "ah." Slowly move the tongue back from the /l/ position and try to replicate the feel of the tongue formerly formed by the use of the tongue depressor.
Note: As the tongue blade turns back, it innervates the superior longitudinal muscles (intrinsic), making a slight trough.