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.
