This is a technique which I've came across quite sometime ago. It's such a fun technique and I've tried it out in my latest layout titled "Catcha Catcha".
I've used some old bubble wrapper and paint some acrylic paint on it before I finally print it onto the card stock. (Tip: Make sure you get a good amount of acrylic paint onto the bubble wrapper before you print it onto the card stock.) You can also try to do it by dipping the bubble wrapper onto the acrylic paint before printing. However, I find this method a little too messy for me.