Put your hand in front of your mouth. Feel the short burst of air for the "ch" sound compared to the continuous stream of air for the "sh" sound.
Put the tip of your tongue behind your top front teeth. Push a short burst of air out.
Keep your tongue in the middle of your mouth. Your teeth almost touch. Blow air over your tongue and teeth continuously.
The "ch" sound
The "ch" sound
The "sh" sound
The "sh" sound