I suggest you read through this chapter once before you try cobbing.
Soil usually contains sand and clay which are the main ingredients of cob. Cob is sand 'mortared' together with clay and strengthened with straw. Imagine the cob as a miniature mortared, reinforced, stone wall. The sand particles, like the stones in a stone wall, provide strength. The clay serves as the mortar. The straw does the reinforcing job of the rebar (reinforcing steel).
You will be sculpting a semi-liquid that will turn to "stone".
A COB RECIPE
The recipe for cob is pretty flexible. Lots of variations work well.
Materials vary so you'll have to make up some test bricks until you find the best recipe for your ingredients.
The amount of straw is hard to quantify. I like to add as much as I can, until each piece of straw is surrounded by the earth mix. You'll need to stop adding straw before there's too much straw, or the cob will fall apart. The more clay in the mix, the more straw you can add. Try it. You'll get a feel for it. Read on for more details.
Soil is made up of various sized particles: tiny stones, sand, silt, clay, and organic matter. To find soil suitable for building material, first scrape away the top organic layer. Use this top soil for nourishing your garden plants. Examine what's underneath. You will be able to tell a lot by feeling and looking at the soil. Can you recognize sand particles and sticky clay?
Take a cup or two of soil from various potential house sites and from various depths. (Soil samples can vary a lot even a few feet from each other.) Take out any stones or pebbles. Put each sample in a quart jar. Label each jar, recording the depth and where the soil came from. Then fill the jars 3/4 of the way to the top with water. Shake well. I mean really well. Shake that baby! Then let it settle. If your soil has sand, silt and clay in it, you'll get three distinct layers.
The sand is the heaviest and will sink to the bottom immediately as you watch. The silt will settle next, more slowly, and the clay will stay suspended in the water for a while then settle on top of the silt.
This will give you a rough idea of the ratio of sand, silt, and clay at each spot on the land. If your soil only has one or two components to it, it will be difficult to know what the layer(s) are. See if you can tell by feeling. Make jar tests with other soils until you find one that separates into three layers. Then you can get to know the feel of sand, silt, and clay.
Start out by making test bricks using just the soil from your site. If you need to import materials, stretch the recipe to use as much of the on-site soil as possible. Too much clay in the cob mix will make it shrink and crack as it dries. Too much sand in the mix makes the cob crumbly and hard to sculpt, and when dry, sand sloughs off if you touch the cob. If you run into these situations, you'll know you've stretched it too far. Depending on your materials, recipes with as much as 50% clay and 50% sand or as little as 15% clay and 85% sand can work.