craftgrrl | Making oversized buttons from polymer clay

Recently I've been sketching up designs for some themed accessories and I was interested in making over-sized buttons to decorate them. I'm thinking somewhere in the region of 7-10cm diameter, just under 1cm thick. I was thinking of the sort of classic button shape with 4 holes and a fat round rim.

The trouble is, I have very little artistic talent when it comes to sculpture. Can anyone think of a methodical formula for making such a shape easily without any judging by eye or requiring a great degree of manual dexterity?