Laying the Foundation...
After selecting the suitable site, you clear your patch of heaven and lay out a rough image of the Earthship's footprint. All that dirt and clay and rock that is being scraped away is what you will use to fill those hundreds of tires.
The house "floats" on this foundation - which typically causes traditional builders to cringe (they want to build deep footers). I am perfectly happy to buck conventional construction wisdom (as I know no better). Fortunately we did not have to put this theory to the test as we were resting on bedrock after scraping off the topsoil.
One thing that the experts (Earthship experts that is) recommend is that you pile your earth right in the middle of where you will be working. This is good advice, saving miles of pushing a wheelbarrow around before your project is finished.
Of course you need to be very careful in placing your first layer of tires, as everything keys off these. Measure and check and double check and measure again.
Then you just keep building up the courses, ensuring they are level and solid. It is also a good idea to level them vertically as you go, avoiding future work when it comes to mudding.