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The individual sounds of l and r are important sounds to distinguish, and they are also important to distinguish in clusters, too.
As in other clusters, remember that you have to make all the sounds in the cluster.
R Clusters
When you make an /r/ sound, remember to pull your tongue backwards, and be sure that your tongue is NOT touching the top of your mouth at all. In fact your tip of your tongue should be in the middle of your mouth. The sides of your tongue are touching the inside of your top teeth.
L clusters
When you say /l/, your tongue is touching the back of your front teeth. Hold it there while you make the sound. When you make clusters with L your tongue has to move quickly to the top of your mouth.
l and r combined
When you are practicing the R and L blends, look in a mirror to make sure your tongue is in different places for the R and L clusters.