Jenalee wrote an answer about Ruffwear Bark'n Boots Grip Trex - Set of 4 on August 23, 2009
If they are "spinning" (i.e. turning backwards) it's likely you need a smaller size, or maybe your dogs paw is not all the way down into the toe of the boot. The strap should end up above the ankle bones, and higher than the "extra toe" on your dog's foot. Tighten them down as tight as you like: it doesn't hurt as long as you don't pinch or fold any skin.
Also: water will cause them to stretch a little, so they need to be retightened after they get wet.
Hope this helps!