Three Easy Steps
1. Fill an ice cube tray half way with water. Freeze
2. Place one rose petal on each cube.
3. Pour one teaspoon of water over each petal. Freeze again.
It's that simple! Here is a quick tutorial.
You could also opt to freeze the entire rose head like the photo below. Spray roses would work perfect since they are a bit smaller in size. Here's another tutorial.
The possibilities are pretty much limitless. For a complete list of other edible flowers that could be considered when making flower petal ice cubes, check out this wikipedia article.