Gordon,
There is the obvious way of adding a carriage return or a shift+return after each letter. But most likely that is not what you are after. In previous Word versions there was an option to add vertical text using a WordArt object. Unfortunately this is no longer the case. You can still use WordArt to fake it, add a WordArt object, type your text, then adjust the width of the box to allow for just one character. You can do this with a one table cell also. Other than that I see no other possibilities, let us hope someone will have better ideas.