So there is no step by step photos for how to do this, but basically:

1. Go for a walk and look for rocks. A beach with those soft, round beach pebbles would work great. But don’t worry, I found these from walking around in my neighborhood here in LA, no need to look far in other words.
2. Rinse of any dirt with water, we don’t want to ruin those pens of yours. Let dry.
3. I prefer to use Posca Markers, but any kind of acrylic paint (either brush or pen) should do. Get creative!
4. Once you are happy with your designs, let them dry and cover with the gloss of your choice, matte/glossy.
5. You are done! Perfect for decorating windows, the garden, pots… Or put them in a nice box and give to a dear friend.