I have used my Valentine Squirrel digi from Pink Cat Studio as the focal point of my cards.
I printed the cards out on Neenah paper and then used a fine point Sharpie to do the doodling. I found this pen gave me the best overall coverage and depth of colour.
Here's a picture of all 7 cards put together. I thought it looked kind of cool!! As you can see the cards are not all identical like most times when you do ATC's so that makes each one a one-of-a-kind limited edition ☺. The idea of Zentangle is to doodle in small compartmentalized spaces. There really aren't any rules, or if there are, I don't know about them. Anyway, I had fun doing these up...doodling is very good for the soul!!