There is a good illustration of purling through the back loop in the Vogue Knitting book (the thick hardcover.) I had to learn how to do this for the Clapotis. It's a bit awkward, but it gets easier with practice.
I used Artyarns Supermerino in colorway 109. I used size 5 needles, and it took about six skeins. It created a smaller Clap, but the thicker garment will be perfect for chillier weather. I plan to make a bigger one with supermerino at some point.
The supermerino was absolutely wonderful to knit! So squishy and cozy. I would make sure you have the right amount of yarn in a specific dye lot. I have seen differing dye lots of the same colorway and they differ dramatically.
I love Mountain Colors, but I am always overwhelmed at the amount of good color choices. I finally narrow it down to two or three colors and spend forever trying to decide which one to get. It takes me just as long to pick the yarn out as it does to knit the thing.