Writing a round Ketubah can be a lot of fun! I still remember my first, written about 20 years ago. I couldn't find a compass to make a large enough circle, so I ended using a large pyrex bowl to trace the outline in light pencil. During those days one had to innovate, and my wife's kitchen was fertile territory.
The circular outline served as an exact guideline to set the limits of the text; which I later erased once the ink dried. I also had someone else working on the illumination, which was done as a separate piece with the center cut-out. It took some planning to interact with the artist, but these eventually dovetailed together with the text sitting perfectly in the middle. Be sure to let us know how yours progresses.