I was working on one but got sidetracked. The vanilla courier is made to accommodate several different quests, and I made mine similar to that one, even though I only needed him for one quest. I made my courier and put him the same cell as the vanilla one, gave him his own container and x marker, etc. After that I made a sort of general courier quest that would start the courier if he had such and such letter, then place him in a village or city and have him greet the character. For that step you would want to look at the vanilla courier and see what aliases he has and what packages he gets when he is activated. It will probably take some trial and error.
If I get some more time I'll try to revisit the courier and make a full write-up in the wiki. There is one in the wiki now but it's really more complex than most people need I think, it was definitely more than I needed and as a result wasn't that helpful. I found that looking at the vanilla courier was more helpful. Also making my own courier rather than using the existing one wasn't that difficult and allowed me a little more freedom, the only downside is not having the "official" courier dialogue with the voice actor and such.