To add an infant to your Delta flight, you can do so during booking or after purchasing your ticket. The how to add infant to Delta flight process varies based on whether the infant will sit on your lap or in a separate seat. Lap infants (under 2 years old) usually fly for free on domestic flights, while international flights may have additional fees. If you need to add an infant after booking, call Delta’s customer service. For assistance, call +1-800-283-2017 and speak with a Delta representative today.