Step one:
First draw a horizontal line and draw a square factory on it.
Step 2:
Then draw several long chimneys on it.
Step 3:
Inside, draw square doors and windows.
Step 4:
Inside the chimney, use horizontal lines to draw its texture.
Step 5:
Also use wavy lines to draw the feeling of smoke.
Step 6:
Finally, paint it with a nice color, and the factory is ready.