By hand.
Look into Typeface creations. These are the conversions of ASCII characters into coordinates, as you need. However; they are based on the individual font, and have only certain scaleability.
I would strongly suggest finding a font definition for something like Helvetica (good luck finding one, they are usually quite impossible to get) and using that information.
I would also suggest an alternative solution to your problem. You are trying to basically reinvent the wheel it sounds like. This is a job for a display generator. However, if your instrument doesn't have one, and you need to make one...well, all I can say is good luck, and be sure to share it with us.