When I am trying a new technique, I prefer to wing it initially, and then research the technique. The yarn above is my first attempt.
I've since done quite a bit of research on the subject and have learned a thing or two.
Overall, my first attempt turned out quite well. I used a one hand method and it became clear to me that I need to use my left hand to hold the twist so I don' t get little tangles as I work a new loop (see middle photo). I also was treadling very quickly and had my bobbin on fast intake resulting in a frantic spin...clearly I need to slow it down next time.
This morning I found this great video by Sarah Anderson on youtube. I really think that this video is THE video you should watch if you are interested in learning to N-ply yourself. She explains quite a lot in a very methodical, logical way all while showing the technique, slowly and clearly.
I'm going to do a few more play skeins with little bits of singles I have laying around before taking it to the shop. My plan is to N-ply the Finn wool I recently hand painted and offer it up in the shop next week.