The first is to create a circular frame that outlines all vertices with the largest circle (see example). After that is complete, you simply fill in the excess area with circles on scale with the layer that it is on.
The second way is simpler, but generally less aesthetically pleasing. To do this, you simply create the smallest circle in the guide, and slowly build larger circles on top of it until you reach the circle you want as your middle point. You then start building smaller circles on top of your sphere until you reach the smallest circle again.
There are multiple guides to make circles on Youtube, and you can also find several on the Minecraft Forums and by simply searching Google.
For extremely large circles you can use the circle tool at http://www.minecraft-craftingguide.com/calculator.html