There really seems to be a lack of a basic tutorial on how to make a trade sign, and there is now one included in the main tutorial above spawn, because it's a concept that just requires visual aids.

DIRECTIONS TO THE TRADE SIGN TUTORIAL:

Type "/tutorial." Don't you feel welcome now?


Turn left.


Walk to the corner of the "courtyard" and follow the sidewalk to this little "shop."


The tutorial looks like this:

 

That's much more thorough than mine. :)