Here are the steps to draw on iMessage:

1. Open the iMessage app on your iPhone or iPad.
2. Tap on the recipient's name to join the iMessage conversation.
3. Tap on the heart icon near the iMessage text box.
4. Start drawing on the black canvas that appears.
5. To get a larger canvas, tap on the upper arrow icon at the bottom right corner of the screen.

You can also use different effects while drawing on iMessage by following these steps:

1. Open the iMessage app on your iPhone or iPad.
2. Tap on the recipient's name to join the iMessage conversation.
3. Tap on the heart icon near the iMessage text box.
4. Start drawing on the black canvas that appears.
5. To change colors, tap on the color dot on the left side of the screen.
6. To change brush size, tap on the brush icon at the bottom of the screen.
7. To add effects like fireworks or confetti, tap and hold down on your drawing until you see a menu appear at the bottom of your screen.
8. Swipe left or right to choose an effect and then tap send.

I hope this helps! Let me know if you have any other questions or if there is anything else I can help you with.