I had found a pattern of a flower online. I printed it out and make a stencil out of it with stencil paper and an Xacto knife. I laid out a whole bunch of my crafting towels and put the curtain on top of those.
Then, I laid my stencil out and painted over it.
I randomly laid the stencil out - again, I love randomness, but this is easy to adapt to what you want. You could choose great colors and patterns for your curtains. Here is my finished project for my guest room:
Let me know if you try it! I would love to see what you come up with too!