Pen is a natural sign, since you simply mimic writing something with a pen (or any other writing utensil) on a flat surface. Hold out your non-dominant hand flat, with your palm facing up, then proceed to pretend writing on it with your dominant hand formed as if grasping a pen and writing. Run your dominant hand once across the other palm, from the base to past your fingertips.
Teach your baby the sign for pen when you see them being curious about what you are writing with.
Teach your toddler the sign for pen along with the act of writing, doodling, or making a list. Children love to mimic adults as far as their motor skills will allow, so don't be surprised if your child tries to doodle something soon after seeing you use a pen!