To make your own simple ring sling, you will need a set of aluminum rings, sold at www.slingrings.com. Please don't use plastic rings or something you rig up from items around your house - these rings hold your precious babe's weight and need to be strong! You will need between 2 - 2.5 yards of sturdy fabric (not quilting cotton or other thin material). There are several simple patterns available online. I followed Maya Wrap's Pattern.
Check these sites for more info on fabric selection and patterns!