Here’s a clear, effective technique for teaching a child with a lisp how to make an S sound correctly. It’s known as the Long T Method, and has been recommended by the well-known SLP Pamela Marshalla.
Here are the basic steps:
Make sure the child can make a T sound.
Now, ask the child to make a T but to stretch out the sound or blow air for longer at the end of the sound.
Show the child how to do this, making an elongated, exaggerated T sound.
Ask the child to make a long T sound again, this time lifting the tip of the tongue up to the roof of their mouth.