here are a couple free patterns.
http://www.sewingsupport.com/sewing-...by/slings.html
I had a non-sewing friend that kept asking me to make her variations since she led a young mommy group and often demonstrated new patterns at her meetings. It's been a long time but I do remember the type of fabric used changed requirements for the rings (fabrics with some give, stretchy, or slick and non-giving fabrics like silks) and I imagine the size of the baby is a factor as well. I only made them for tiny munchkins but I'm sure there's some great info online.