Freehand
Or you could paint around something circular.
Like how you draw a circle for things using a food can.
Find something as small as the circle you want, and trace around it.
Micron pens come in handy here, if you cannot get a line thin enough of paint.
If you are not wanting a hollow circle, but a dot...you can dip a pin tip (like paper clip) in paint and touch it to imprint the circle/dot.