|Dogs have heads that vary a lot in size and shape|
For the skull width, gently feel between the dog's eye and ear. You should find a bar-shaped bone: the zygomatic arch. This projects out from the skull over the jaw muscles on either side of the head. Since dogs have rather strong jaw muscles, this will always be the widest part of the skull. If you're curious, check it out. Feeling under the ears will reveal a smaller skull width than what is seen at the zygomatic arch. To measure the skull width, don't simply lay a tape measure from zygoma to zygoma. Since the skull isn't flat, this will give you a falsely large measurement. This is why calipers are usually used to make these measurements. You can jerry-rig a set of calipers using two straight objects and a ruler, but this is difficult and there are easier ways to do it, which I will discuss later.
For the skull length, find the body protrusion on the back of the skull: the occiput. It should be fairly simple to find, sitting between the ears and often very close to where the head meets the neck. The prominence of this can vary a lot by breed, so if you have a dog with a very round skull you may not be able to find it. If this is the case, take the measurement from the farthest protruding part of the skull you can find. Measure from this point to the tip of the nose.
What would be the easiest way to get these measurements? Well, a lot of dogs would probably not be very happy with you messing with their head, so an alternate method of finding these measurements is using pictures. If you take a photograph of your dog's head from above, you can use this to measure the CI. Units don't matter, so you can use one measurement to figure out the other. Here's me doing this with Ebon:
So, can you calculate your dog(s)' cephalic index? If you do, let me know what you come up with.
The First and third images are from Wikimedia Commons. The first is under a Creative Commons license and the third is copyright free. The second image is mine.