If your soil has a lot of sharp rocks in it, you can gradually toughen up your feet or give in and wear shoes. Smooth soled ones work best because the cob doesn't stick to them so much. Another option is to sift the dirt through a 1/2 inch wire screen. This is a lot of work and puts dust into your lungs. Wear a mask if you do this! Breathing dust is bad for you! Be aware that the clods that won't go through the sieve easily are the hardest and best pieces of high clay-content soil. These can be forced through the sieve, or crushed and then sieved. Or these precious little lumps of clay can be soaked overnight, then put through the sieve wet or mushed up by foot. You can save the gravel that you end up with after sieving for your drains, or put it back into the cob after you've finished mixing it.
When your mix begins holding together in a giant loaf when you turn it with the tarp, it's time to add straw.
When you add straw to the mix, it will dry it out quite a bit. If the mix is too dry to add lots of straw, you can either add less straw or add water. You will soon get a feel for the consistency you're after. Remember this ancient knowledge is imprinted in your cells. Trust yourself. Experiment.
Flatten the cob loaf down by treading on it. Grab an armful of straw and sprinkle some onto the mix. Step on it so it sticks into the cob a little and pull the tarp up, making a cinnamon roll of dirt-dough and straw-sugar. Tread on it until it's flattened out into a pancake again, and add more straw. Make another cinnamon roll. Do this three or four times until you have enough straw in the mix. Too much straw will cause the mix to fall apart. Try adding a lot to a small mix so you can see what too much straw feels like.
A cob mix that's too dry will be crumbly, won't want to stick together very well, and won't take straw well. Add water and stir until the mix is the consistency of cookie dough. Too much sand or too much straw can also cause a crumbly mix.
When you build with a mix that is too wet, you'll see that it won't keep its shape because it can't hold up its own weight. It will start to bulge out on the sides of the wall. The technical term for this is "ooging". (All cob oogs. The wetter the cob, the sooner it oogs.) If it's too wet, add a little dry sand, dirt, or straw and tread it in. If you add a large amount of dry stuff, use roughly the same ratio of dirt and sand as your original recipe. You can also dry a mix by treading it into a pancake so the air can get to it, and letting it sit for a few hours or overnight. You'll quickly get a feel for it.
Because it is so much easier to mix the cob if it's on the wet side, you may want to do it that way and let the mix sit for a day or two to dry out before you put it on the walls. When mixes sit, something magic happens and they become more elastic and easier to work. You'll get a feel for how wet to make them and for how much they'll dry during a night. If they're too wet, uncover them and let them sit a little longer. Once you get into the swing of cobbing, you can make mixes on all your tarps at the end of the day when you're warmed up. In the mornings you can get inspired and warmed up by cobbing onto the walls.
Curing is not necessary. It's fine to make a mix and put it directly onto the wall. I suggest you try letting a batch of cob sit, though, to see if you can feel an improvement.
Avoid leaving the mix covered for too long because the straw will start to rot and stink, and lose some of its strength. If you do this accidentally, it's no big deal, put it on the wall anyway. It will stop stinking as soon as it dries.
In the old days in Europe, farm animals were often used to mix the cob. The ingredients were shoveled into a circle around a post. The horses or oxen were tied to the post and walked round and round treading the cob mixture.
In recent times people have used various machines to make big batches of cob, and to speed up the process. Caution: if you let a batch of cob sit wet for too long, the straw will start to rot. It's a good idea to only mix big batches when you know you'll have time to get the cob onto the wall fairly soon. One strategy is to mix a big batch of the clayey soil and sand and add the straw to only the portion of the mixture that you know you can use within a week or so.
Cement mortar mixers will mix cob if you can handle the fumes and noise. As with manual mixing, the clayey soil and sand is mixed first, then the straw added. The cob has to be mixed wetter than with other types of mixing to allow the cement mixer to turn without too much stress. This means the mix may have to sit a day or two to dry to the right consistency. Straw can be added manually to the mix.
Tractors or four wheel drive vehicles can make quick work of the cob mixing job. I've seen the process happen unintentionally in peoples muddy driveways. Simply pile the soil and sand and drive back and forth over them. Add the straw after the other ingredients have been mixed well, then drive over the mixture again until it's well stirred.
Heavy earth moving machinery can be used to mix up big batches of cob.
Front end loaders are great! They can do the job of compiling and mixing the
ingredients. The finished cob mix can be scooped up in the loader and raised
to the height of the wall. You can climb into the loader and hand-sculpt the
walls without having to lift the cob up onto the wall by hand.