To sign count in ASL, your non-dominant hand should be flat with palm facing up to form a base. In your dominant hand, take your thumb and index finger together as if they are making an outline of a coin. Then run your imaginary circle along you non-dominant hand.
For children that are interested in numbers, sign count before you do any counting. As your baby progresses, you can use count to prompt them to start counting. At first, counting can just be like a song, with you repeating the words and performing the signs. But as your baby advances, you can use the sign to prompt them to count some small groups of things.
Use count together with the numbers signs. If your child is interested in numbers, you can improvise counting games everywhere you go. When you see a group, just ask your baby to count. In their first few years, most babies will not be able to count beyond 'three', but laying the foundation for counting now will make the concepts easier later on.