A ring sling is a long rectangular piece of cloth threaded through two rings. It is easy to adjust and transfers easily between caregivers. A ring sling can be used with a newborn through toddler and is a fantastic choice is you can only select one carrier.

Instructions (from other websites)
Videos:
Tummy to tummy hold
DIY: Please use caution in selecting rings! Do not use craft rings.
Troubleshooting and Safety:
Pictures: