"How do I slip my stiches onto the spare yarn, disconnect the yarn, knit the other pieces, and reattact the yarn to the stitches, without damaging the last stitch I knit?"
First, you will need a separate ball of yarn. Any yarn will do--it will be discarded later.
Next, you will need a carrier for the "spare" yarn. You can use a yarn needle, a floss-threader, a twist-tie bent in half, a crochet hook, or anything that will take the "spare" yarn through the stitches on your needle.
When you are ready to do this part, follow these steps:
1. Cut the working yarn leaving about 12" of yarn attached to the back neck.
2. Using a separate yarn about 24" long, thread the yarn through a carrier and run it through all the stitches still on the needle.
3. Put the two ends of the yarn together and tie an overhand bow knot (one you can easily slip out). http://commons.wikimedia.org/wiki/Category:Overhand_knot_with_draw-loop
That's all there is to it. Then complete the fronts (and maybe the sleeves) according to the pattern. The instructions will tell you how to join the shoulders. You will then pick up stitches for the rest of the hood from the two front pieces. At that time, the pattern will instruct you on how to join the working yarn. Meanwhile, don't worry about the back piece unravelling--it won't. The spare yarn run through the live stitches will prevent that from happening.
Other methods of "holding" live stitches while you finish other parts of the pattern include using a stitch holder--which looks like a large safety pin (http://3.bp.blogspot.com/_kyYaMHB4tGw/R7i8Ju8CBCI/AAAAAAAAAlE/EFt0tU-vnK4/s400/knit+holder+1.jpg ) or using a double-pointed or cable needle with point protectors on the points to prevent the knitting coming off.
