To make your own ring sling you will need to purchase:
1 3/4 - 2 1/2 yards of fabric
1 pair of sling rings
As for the length of fabric, it all depends on how long you want the "tail" of your ring sling to hang down, as well as how big of a person will be wearing the sling. For example, I am a small person and I like a short tail, so I usually only buy 2 yards of fabric. If you want a feel for how long you should buy, wrap the fabric around yourself with extra fabric for the tail.
As for the type of fabric, the absolute best is something that is 97% cotton, 3% spandex. The little bit of stretch is really nice. That said, I very rarely buy this because I mostly base my purchases on how cute the fabric is! The type of fabric I buy most often is 100% cotton or a cotton/poly blend. You want a fabric that is 54/55 inches wide - but if you love something wider, get it and just cut it down to size. Also, once you've experimented, you can make a gorgeous, fancy sling out of duponai silk - but I wouldn't make that your first try.
As for the sling rings, I only purchase mine from www.SlingRings.com. These are great rings and made specifically for slings. You can buy all sorts of colors. I always purchase the aluminum rings. Medium size is good for a normal weight fabric. If you have a thicker fabric, I would purchase size Large.
That should be it! Watch the video below for the how-to. If you have questions, PLEASE post them. This seems clear to me, but may be mud for someone else.