I’m drawing the ellipses on the vase while I’m waiting for my canvas to arrive. I calculated the angles of the ellipses at the top and bottom of the vase, using a string and protractor. (See Portfolio and Jewelry: Drawing for more details on drawing ellipses.) They measure 11 degrees at the top and 23 degrees at the bottom. I interpolated the angles of all of the ellipses in between and indicated them on the drawing. You can see that in the photo below. After I know the angle and the length of the major axis, it’s simple to draw the correct ellipse with a string and two pins. I transfer the ellipse drawing from the tracing paper to my drawing by flipping it over, laying it on my drawing, and tracing the lines.
With the ellipses in place, I can start to draw the designs on the vase. It’s tricky getting a regular pattern to appear to wrap around a cylindrical form. Repeating patterns will look closer together as they turn away from you. I could calculate how they would appear precisely using geometry, but it’s not worth the trouble. I can eyeball it!
Small details won’t survive the underpainting. I’ll indicate them as best I can, but I always end up redrawing them with my brush in the final layers.