To sign my or mine, simply take your flat, open dominant hand and lay it on your chest. A flat, open hand signifies ownership depending on the person you are directing it to. So if you want to teach the sign yours to your toddler, simply make your flat palm face him or her, followed by the sign of the object in question.
Teach your toddler the mine sign when they are ready to understand the concept of ownership.
This is a useful flash card taught best in context, in conjunction with the actual toy, food, pet, or person that your toddler can lay claim to. "Is this gift mine? Thank you!"