Make the bottom of the wall wider than the top. I usually make the interior side of the wall pretty straight up and down. It's a matter of taste. If you want your walls to grow organically, use your common sense. You can angle the wall for the first foot or so and then make it straight up, imitating a tree trunk or the interior of a cave.
The wall has less and less weight to support as it goes up, so it doesn't need to be as thick as it gets nearer to the top. You can taper the outside of the wall. Tapering cuts down on the amount of cob you'll need and makes the house that much lighter. The angled shape is a stable shape. Taper two inches for every three feet of height.
On average, a wall will need to be 9 or 10 inches wide at the top, tapering out 2 inches for every 3 feet of height. If your wall is 9 feet tall, start by taking the 9 inches you'll want at the top and add 6 inches (2 for every 3 feet of the height) which comes to 15 inches. Add another inch or two for good luck. So that's 16 inches at the bottom of the cob wall and at the top of the stone foundation. If you're planning two stories, use the same formula. These calculations are vital for deciding the width of the foundation.
Cut a wedge measuring 2 inches x 3 feet out of styrofoam or wood and tape it to one side of the level. Leaning the wedge side of the level against the outside wall, get the bubble to read level. The edge of the wedge will then show you the taper you're after. Cut off the bumps on the wall to form the plane you want. Use the other side of the level for your interior vertical line.
As you build, you will be creating the vertical planes on the sides of the walls that you will always see. As you add cob, keep an eye on both the inside and outside plane of the wall. Cob has a mind of its own. It's surprising how hard it is to keep the walls growing the way you want them to by just using your eye. If you don't check a level often, it's easy to build a lot of unnecessary width into a wall before you realize you're going wonky.
Cobbers tend to push the wall away from themselves as they massage new cob onto the wall. This sometimes distorts it towards the other side of the wall from where they're working. It's helpful to cob with a friend, with one person inside the house and the other outside, working opposite each other, keeping an eye on the vertical plane on 'your' side of the wall.
When working on a tightly curved section of the wall, work mostly from the outside of the curve. Cob curves tend to lean outwards as the wall goes up.
If you keep adding a lot of cob at once, it will eventually start to oog or bulge out on the sides, pushed out by its own weight. When that happens, stop cobbing in that spot and move on to a different section of the wall. If you have a big house-raising party and you manage to get to the point where the whole wall is ooging, pat yourselves on the back, make mixes in all the tarps to cure for tomorrow, and call it a day. Unless you live in a very humid climate, the wall will be dry enough the next day to chop off the oog and keep cobbing.
It's much easier and stronger to chop off cob than to add it to a dry, or even a partly dry, surface. It's better to make your walls too thick and bulgy than too thin and dippy.
The walls can be checked with the level and trimmed to give you a more exact shape. Do this in the mornings before you start cobbing. A machete, meat cleaver, or spade are good tools for shaping the walls. Chopping cob will dull your tools, but they seem to work just fine without sharpening. A 2x4 held on its side and scraped across the wall helps to make a flat surface. When you're doing the trimming, stand back occasionally and look at the walls from a distance to give yourself perspective.
The cob that you chopped or scraped off can be re-wet, retread a little, and recycled back into the wall, or thrown into an almost-finished new mix and tread in a little.
Another totally different approach is to build the whole house, leaving the sides of the walls all lumpy and messy. Then after the walls are built, wet them down really well and carve them into their final shape all at one time. A hoe or shovel are good carving tools to use along with your big knives.