1. Open materials editor
2. Duplicate a similar material (ie, for grass use an existing grass material, for road use road, for a sign, we will use wall.
3. Remove the extra stuff and just make is a single image.
4. Add your sign texture. and that's it.
Use wall to create it, you might have to fiddle with the texture maping in the wall's material editor. ie height width offset etc.
(Don't let the image confuse you, I just pasted each new view on top of the other. you will only see 1 dialogue at a time)