Goal: To teach that the word needs an "engine" to start moving.
Instruction:
The Analogy: A car cannot move without the key turning the engine on. The first sound is the key.
The Visual: Hold a "Start" button (draw a green button).
The Action: Press the button hard while saying the first sound: "T-T-T... Top!" or "Sssss... Sheep!"
Elongate the first sound to make sure it's there before finishing the word.