How big are the girls? there is a free pattern (for a regular sling) on the Maya website (click on the site map, then "low cost alternatives"), and I use those instructions to make doll slings. It's about the easiest thing out there, IMO.
For a toddler/young child, 1/2 yard of fabric, 45 inches wide should be plenty. I like to hem all sides, just because it looks neater, lasts longer and doesn't take that much time, then fold it as the maya instructions show, sew on small rings (I use rings that come 5 in a package for about a dollar--they are probably 1.5 inches diameter (outside measurments), thread it, and you are good to go!
Since safety isn't an issue, the sling doesn't need to be snug, so thread it to a position that is easily slipped on and off, and just leave it there! As the child grows bigger, it is still somewhat adjustable, so it will grow with them.
This link should get you right to the maya sewing instructions:
http://www.mayawrap.com/sewSling.shtml
HTH!
