I was working with my daughter and helping her with the "s" sound (she often says "th" instead of "sss") I tried to help her understand to put her tongue behind her teeth instead of between her teeth. I showed her using my mouth, I held her in front of a mirror, I even tried to move her tongue for her. She had many wonderful attempts at it, but she just wasn't putting her tongue behind her teeth. Because she was so motivated to speak and play with me I had more opportunity to work on this than I usually do with other children with autism.
I tried breaking the skill further down and encouraged her to wiggle her tongue, this even seemed challenging. I broke it down further and asked her to just stick her tongue out, she did but with her mouth wide open. To sharpen the skill I had her do it with her mouth closed. She once again was challenged with this. She seemed to start to lose interest so I switched the game up, I had her making vowel sounds that she can do easily. She repeated funny sounds saying me, mo, mu, mi, may, and then I threw in a tongue wiggle. This time she did it!
She still has a way to go until she is saying her s's consistently, but it was so nice to remember all the little intricacies that go into talking. The dozens of muscles we have in our face, our lips, our tongue, our teeth, our voice box, our breathe, then of course also the brain neurons all firing at exactly the precise moment. Making any sound at all is a minor miracle :) Let's remember how complicated speaking is and celebrate every little step our children take!
If your child has a challenge with clear words trying breaking it down into the smallest skills you can think of and build up from there. Celebrate any and all attempts and remember to have fun!