Cob is mixed with a combination of stirring and compression. In my experience, the best way to achieve this combination is with feet and tarps.
Clear a few places at the site for mixing areas. The closer they are to the foundation, the less energy it'll take to get the cob to its destination. Inside the building is a good place for at least one mixing area. Leave enough room to walk around the mixing sites easily. Use your brain to minimize work. If you've imported ingredients, pile them in sensible places to streamline the process. For example, if the sand pile is two steps closer to the building (but not crowding walkways) you'll save miles of steps during the building process.
If you need to add sand or clay to your soil, you can measure out proportions by the shovel full, or by the bucket full until you can do it by eye and by feel.
Break up the clods with your tamper or your shoes
If you are adding sand, shovel the sand onto the tarp and stir it into the crushed soil until the two are well-mixed. You can do this by pulling up the edges of the tarp and rolling the ingredients around on the tarp.
If the soil you are working with is so sandy that you have to add clay, you can either add clay soup or add dry powdered clay to dry sand. Wear a mask when adding and stirring dry clay to your mix.
Stirring and treading well is a big part of the magic that turns your ingredients into cob.
To stir, stand on one edge of the tarp and lift up the tarp on the opposite side of the mix. Pull it towards you, turning the mix over onto itself. Don't lift the edge of the tarp at your feet or push the mix to turn it. That's too hard on your back. The more you roll the tarp, the faster the cob mixes.
Then add water. You're after a cookie dough consistency. Tread on the mix with your feet. It's easier to feel the mix with bare feet, and for some mysterious reason, bare feet mix the cob faster than shoed feet. Don't despair, your feet will return to nature and toughen up quickly.
Stir the mix often by pulling up the edges of the tarp, while you're treading. It's impossible to tread the mix too much, so go for it. You'll notice how it changes plasticity as you tread on it.