Leave plenty of time! Building a foundation is a delightful puzzle. Well, that depends on what kind of person you are. As far as I can tell there are two kinds: the ones who love stone puzzles and the ones who hate them. I've noticed that some days I'm on and others I'm not, so give it more than one day to decide which kind you are. If you fall under the second category you might choose to do some other kind of foundation, or find a friend or two who love stone puzzles.
It makes sense to spend a lot of time looking at the rocks to see what shape you need and which rock fits where, instead of using your muscle to lift and fit them together by trial and error. Take your time. Hurrying and rock work are opposite concepts. Rocks have been around for a long time and they like to move slowly. Stonework at its best is an incredible meditation.
Some people say to wear sturdy shoes and gloves for stonework. That won't help nearly as much as keeping your soft little fingers and toes out from between a rock and a hard place. If you're working with someone else, be careful of each other's fingers and toes, or work on separate sections of the wall. Be mindful!
Remember to tamp!
The bottom is a good place for the biggest, heaviest stones. Find where they want to sit firmly and snuggle them up close to each other. Keep in mind that you are also creating the base for the next layer of stones to rest on. Remember to make the bottom of the foundation wider than the top. Moving stones around at or below ground level is especially hard on the back, so take it easy. Lay stones all along the bottom of the foundation trench. (See page 50 to help you decide whether or not to use mortar.)
Cover each vertical crack of the preceding layer with a stone.
Stones have a mind of their own. They seem to know where they want to be. Don't argue with them: you will lose every time. Make sure each stone is happy by standing on it and rocking it. (Is this where the word "rocking" comes from?) If it wiggles, move it until it's secure, or use a small strong rock as a wedge to stabilize it. It is easier to stabilize a stone that is resting on three points than on four. Long stones that run the whole width of the wall every once in a while add strength. Continue along, carefully fitting each stone into its place. Fill any empty spaces with small stones: they help hold their big sisters in place.
Patience and practice are very useful at this point. Remember to stand up often and stretch your back by leaning backwards to re-squish the discs between the vertebrae back into a rounder shape. Drink lots of water! Too much rock work at a time is too much rock work. Take it easy. Warning! For the folks who love puzzles this can be very addictive.
As soon as you get above ground level, the shape of the foundation will be permanently visible. Stand back and look at your artwork from a distance. Do you want an interior wall that is perfectly vertical all the way down to the floor? If so, build the foundation so it will line up with the vertical plane of the wall. On the outside, do you want the same angle as the exterior wall taper (2 inches in 3 feet) or do you want the super stable look of a steeply tapered base? Build accordingly.
If you want to see exposed stone, use ones that create the look that you want to see on the sides. Save the prettiest stones that you want to see forever and use them above ground near the entrance or patio areas. To highlight these you can stick them out a little from the plane of the rest of the wall. Use your artist's license.
Continue building until you reach the height you've decided on. The top needs to be rough so the cob has something to hold on to. It's best if the upper facets of the top layer of stone angle either into the center of the wall or into each other. If you are using mortar, do not make any places on the top where water will pool.