The easiest way would probably be if you started with a dried clay powder (some of the artist/pottery supplies sell it this way), and just mix everything dry, and then add water.
If you end up with a slab of moist clay, you might want to try smushing it into sheets, and then forcing it through a screen or something to break it into granules. Or maybe you can add enough water to make a slurry. I don't know, I haven't had to deal with this, but it sounds like it would be more difficult then the powdered form.
I've used kitty litter before, and it pops up every now and then on the forums.
According to Walstad, the soil should provide plenty of iron, so I don't really know if it's something you need to worry about adding to a soil mix.