Well, First this is a forum in English. Tries to write in that language if you want a quick answer.
Second there are several ways to do what you want the easiest and cheapest is:
1. Precomponer your components with the flag question of who respects you Las transparency.
2. Create a solid the size of your composition. And apply a fractal noise (fractal) encouraging the evolution and displacement in the direction in which "the wind" this will be your displacement map.
3 in the precomp. With your flag displacement map applied it in Spanish or "displacement map" Sete as the solid source with the fractal noise. desactiva la visibilidad del solido y listo.
You can also use plugins payments are better but take longer to Renderer (if renderer does not render as many say, Is the same as starting and initializing) every frame. I hope that helps you serve. Greetings