|Yea, I, Serpie (Don't ask, just my new nick), has don' a tutorial.. Let's go :
draw an isometric line
Draw a circle between the two pink points
scale the width of the whole circle ! (cyan)
with green (circle).. my values (without zoom) 69 pxl
draw a, i say "even" circle
in my picture 70pxl big, dunno why it is 1pxl more than the circle..
draw the isometric circle (Green) in the middle of the 'even' circle (blue)
delete the lower half of the 'even' circle (blue), and when you want remove the back lines of the green circle, but i think, its better with them, when u want to do some stuff with transparency glass .
a little info :
I did this tutorial in german in 10 minutes, it's also easy to draw it
When questions left, just ask me, just send me a private message
PS.: Shading is at the first your problem