The poster references 300, specifically this scene:
But actually I kinda had the scene on the brain a little before that, having recently discovered the "This is Sparta!" meme.
- Think of a location that would have sufficient cultural recognition and that can be simplified into a minimalistic image.
- Tagline brainstorming.
- Searching for appropriate reference images on Google image search ("looking down" and "kicking" in this case).
- Drawing out the silhouettes on Photoshop.
- Submit. :D