Somewhere nearby a bobcat (preferably) is mixing a clay/sand/shale mixture with about 4% cement and enough water to make it moist, but not too wet. It is hard to advise on the correct moisture content, telling someone it should be x% isn't much help on the job.

Our rule of thumb was...   If it falls apart when squeezed in your hand it is too dry, if it doesn't shatter when the ball is dropped from shoulder height, it is too wet.  

After a while you get to know by the colour, it darkens as it gets moist. Really it is a matter of trial and error, sometimes we had to add more water, sometimes it was a bit wet. If it is too wet, the material displaces rather than compacts. If it is too dry, I guess it will be a bit crumbly afterwards since the cement might not go off. As long as the moisture content isn't too extreme in either direction I doubt you would have any problems.

This mixture is then spread evenly along the bottom of the box (which in our case was sitting on concrete strip footings) to a depth of around 200mm. Using back-fill tampers it is then compacted.

These rammers are like a hydraulically powered pogo stick. They are fairly easy to control and do a quick and thorough job of compacting all the material down to about 50% of it's original volume.

The material changes its "feel" as you ram. Too hard to describe, you can tell when it is right. Another layer is then thrown in and compacted and then another, until the formwork is full.

Don't forget to take out the spacers as you ram...we have a couple of 25mm x 25mm hardwood "oddities" in our walls. Unless I pointed them out you wouldn't spot them, but too many could be either a distraction...or perhaps, if done properly and intentionally, a feature.