I have a Moby Wrap which I absolutely adore but have recently started "making" them for friends. The difference is that I bought a 12m roll of fabric on sale for $15 which will make at least six wrap carriers & a few slings compared with $150 ish for the Moby.

The recommendations are:
* 4.5m length (but you can cut it shorter if you're small)
* 50cm width (usually the fabric you buy will be 1.5m width so you cut it in thirds & have three )
* jersey is recommended (like t-shirt material), when it's cut it curls a little & then you don't have to edge it
* if you get jersey, make sure it stretches only one way
* woven cotton materials are also ok, but you have the edge them with an overlocker or hem
* buy your fabric in one length, sewing two pieces gives you a weight bearing seam
* it's recommended to have fabric that doesn't have a "wrong" side, but I have used a gorgeous striped fabric with a wrong side & you son't notice it
* try to get 100% cotton fabrics because they breath better.

If you need anymore help let me know