Are you talking about foil or latex balloons? For foil balloons you could make yourself a "stencil" of the desired letters (using cardboard or heavy paper), place the stencil on the flat balloon, outline the stencil carefully with a thin balloon marker (see our tip about writing on foil balloons), and then work your way to the inside of the letter.
For latex balloons, I am afraid I have no good idea how to do this, unless you would use a balloon printing service.
If you make a purchase through a link on my site, I may receive a small commission at no extra cost to you. I only recommend products that I believe to have good value (affiliate disclosure).