The affricate “ch” is a later developing sound that can be tricky for a lot of kids to figure out how to produce correctly because it’s actually two sounds in one– /t/ plus “sh.”
What you need:
a child who can produce /t/
a tongue depressor if tactile cues are needed
a mirror if visual support for lip rounding would help
Step 1:
Tell the child to produce a /t/ sound. Praise them for the good sound.
Step 2:
Next, tell the child that you want them to hold out the /t/ sound to make “ts” (bonus step, you can use /t/ to elicit /s/ if the child is unable to complete this step). Have the child repeat “ts” a few times and praise them for it.
Step 3:
Next, instruct the child to make a “ts” with rounded lips. Have them say “oo” to feel their lips round and have them watch in the mirror if they’re struggling. Do 5 to 10 repetitions until the child is able to repeat “ts” with rounded lips easily.
Step 4:
After the child is able to produce /ts/ and round their lips, tell them “now we’re going to start the sound a little farther back in your mouth.” The target position for “ch” is to have the /t/ portion of the contact just behind the alveolar ridge, or the “drop off” behind your gums.
Step 5:
The final step is to shorten this long “ch” production by telling kids to do the same thing, but stop the sound sooner. Model “ch” and see if their imitation skills have improved.
Conclusion
Once they have an easy time producing “ch” in isolation, they’re ready for syllables. Start with mid and back vowels that are either lax or rounded to help ease into the transition from the new “ch” production to a vowel.