This method works great for trying to establish an /s/ for the first time when a child has some kind of substitution error, meaning they make another consonant sound any time they should be making the /s/, such as a “th” or /t/.
What you need:
A child who can make a /t/ sound.
The Process:
Cue the child to produce their /t/ sound and praise them for it. You can do this by having them imitate a /t/ a couple of times. Tell them you like how they’re keeping their tongue behind their top teeth.
Next, ask the child to make the /t/ sound again, only this time push the air out longer. When a speaker does this, they effectively make a “ts” sound. Model this for the child by doing it yourself a few times.
If it is, say “great job! Do that again!” Have the child repeat this sound a few times. Then ask them if they can make a REALLY long /t/ sound.
At this point, the sustained, long sound they’re making should sound like an /s/. Reinforce this great long /t/ by praising the child, then have them repeat a bunch of times, maybe 5 to 10 times in a row.
Then you can let them in on the secret—“hey, doesn’t this long /t/ sound remind you of /s/? Let’s try a few words.”
If they’re successful with the /ts/ words, you can expand and work on more of these. Then, after a few days in a row of practicing final “ts,” see if you can introduce some words that start with /s/.
Conclusion
More repetition leads to more automaticity, and through many repetitions, the muscle memory for the sound becomes ingrained and allows the child to have more success when trying to “over-ride” the old error motor pattern.