The basic difference in pronouncing [L] and [R] are similar to the [D] and [Uh] or ‘schwa’ vowel sound. Both the [D] and [L] touch the ‘ridge’ of the mouth, about 1/2 inch behind the front teeth. With the [R] and [uh] or schwa vowel sound the tongue never touches the the roof of the mouth at all. The actual shape of the [R] letter may help you to remember to curl the tip and sides of the tongue in a semi-cup like shape, but still keeping the tongue in the air just under the roof.
Then practice such contrastive words as:
At the beginning of the word: row // low; road // load; wrong // long; wrist // list; read // lead
Mid-word/consonant blends: fright // flight; brew // blue; frame // flame; fry // fly; broom // bloom
At the end: far // fall; bar // ball; are // all; roar // roll