Use these pictures as you talk about the difference between /r/ and /w/. You can even point out the difference between the letters R and W.  Ask, "which one should I point to"?  Make sure you review what all the pictures are several times before you do this activity.  

Read                                                                          Weed

Rain	Wayne
Ring	Wing

No need to buy worksheets for speech sounds.  Go to this link to get free pictures to practice sounds.

http://mommyspeechtherapy.com/?page_id=55