Mark snap positions and facial features on the front (fabric side of the PUL) with a Mark - B - Gone fabric pen, being sure to line it up with the face circle marked on the back laminate side of the PUL. (justcraftyme.com)
You may need to use a yardstick to help, and you may also want to mark a straight line down the middle of your fabric with a washable fabric pen or with sewing chalk before you begin. (abckidsinc.com)
Embellish them with a Sharpie or any kind of fabric pen. (cityfarmhouse.com)