I had thrown forms related to this quite a few times and knew the scale and proportion that I wanted to achieve.
First, the clay has to be conditioned to be at the correct stage of dryness for cutting, decorating, and building. I undress the block of clay the night before, and leave it out for 4 or 5 hours, depending on whether or not I have a fire in the studio. In summer, it takes much less time. I just want to be sure that the clay is stiff enough that it won't collapse when I start assembling my forms. It's also much easier to do the stamping, rolling, and other surface decoration when the clay is a bit firm.
The next morning, I cut the block of clay into 3 or 4 slices so it can dry further, and will be easier to put through the slab roller. I use Slab Mat paper so I don't have to remove any canvas texture from my work. If you don't have a big block cutter like the one in the image, there are still lots of ways to cut nice slabs. If they are going to go through a slab roller eventually, it doesn't even really matter.
Using a drafting compass, I create conic patterns for my teapot. I am using stiff sheet PVC for my patterns - permanent and durable for studio use. On the pattern pieces, I will record the name of the piece, how many pieces there are, and assign a number to each piece. That really helps for storage and retrieval. I have more than 100 different patterns floating around my studio. I do try to keep each one in a labelled zip lock bag.
Here's a little mock-up of what the form will be. This step is an essential part of pattern making - it will allow me to assess the scale and proportion before whacking into the clay. As you can see from the picture, I have already reduced the diameter of the finished piece a bit to conform to my internal vision of the teapot.
Pattern making in the clay studio is a skill that can be learned. I got a major head start on the process through decades of sewing practice, where I made most of my own clothing patterns. Any craft that requires estimation and visualization will help with pottery making....
Here are the finished body patterns, ready to cut. The clay is about 1/2 centimeter, or 3/16" thick.
In my next post, I'll decorate these two components, and assemble the body of the teapot. There's a previous post that goes into considerable detail around building a spout with a mandrell, so I won't revisit that step.