I don't know about transfers, I think anything that small would be too annoying for me to use, clumsy git that I am.
However, I have had success with painting it freehand - the trick is to use just the tip of the brush, and make sure your paint is nicely watered down, thin enough to easily spread yet not too thin that it doesn't stick. And also to not try and write the whole letter in one go, break it down into individul strokes, for example an A would be one stroke down at an angle, another at the opposite angle, and a horizontal stroke ( like / \ -), or an S would be broken into 3 angled strokes.
Alternatively, there are very fine point paint pens available, and I know there are a lot of people that use them instead, and it's a lot easier to write on than it is to paint on, so that could be another avenue to pursue.