I have some rather detailed drawings I paint/draw onto some of my clay items. I've tried a few tools but haven't yet come up with something that is easy to use and creates a pretty consistent fine line. Currently, my best bet is the Mayco Designer Line (https://www.dickblick.com/products/mayco-designer-liners/?clickTracking=true&wmcp=pla&wmcid=items&wmckw=30488-2013&gclid=CjwKCAiAxarQBRAmEiwA6YcGKLDgI7HBdZTzv_Rr2B3JvH5Llnqkbl8iyhM8h12C-BcvR10eB8kOjhoChSUQAvD_BwE) but my "sque
You might want to try a Ruling Pen. It's an old graphic designers tool that works kind of like a fountain pen. You can adjust it to any width and use it with any liquid, including underglaze. The learning curve is to mix the underglaze with the right amount of water to match the consistency to the width of the line. It takes some practice. But there's no squeezing involved! I use this when personalizing wedding gift platters, to write the names of the couple and the wedding date.