The thing is, this dress needs to be made with stretch knit fabric in order to lay right and wrap right. Stretch knit fabric at JoAnne Fabrics costs $9-$12/yard. You asked what yardage you will need... For the straps alone, you will want them to be about 1.5x your height. So if you are 5 feet tall, you will need 7.5 ft of fabric--2.5 yards. You could maybe use the remaining fabric for your skirt if the original fabric was wide enough, otherwise you will need to buy 1-2 more yards of fabric, depending on the size or style of your skirt. The good news is, for this style of dress, you don't have to use expensive stretch knit fabric for the skirt, just the straps, however the straps alone will still run you $22 if you don't have a more affordable place to obtain your fabric.
This video helps in that it shows how to sew it together for once, and some clever shortcuts like not cutting the straps in 2 until you've sewn it on (which doesn't help if you wanted the straps to overlap slightly), and plenty of ways to wear it once made. But if you are looking for more details, words, and tips, see Kristina's blog post entitled: "The Convertible "Infinity" Dress: How it almost defeated me, and what you need to make one"

I just bought 6 yards of a pretty blue knit at Handcock fabrics for $18. You just have to keep your eyes out. I am so miffed that Walmart quit carrying fabric thought they were like half the price of everyone else!!

Thank you so much!