I am not sure I understand your question. This tutorial requires two pieces of fabric for the top and under side. Do you mean closed as in once you finish sewing the two pieces together and turning them inside out, sealing the hole that is left? Please let me know exactly what you mean! I may be able to describe it simply to you or I can make a video myself and put one up for you Just let me know what you mean!
Sorry for the super late reply (again) I have been busy!!! I don't really have time to make a video, but I will do my best to explain it!
After you turn the collar inside out and press it, the ends of the fabric from the hole you pulled the collar through - poke the fabric inside the hole and press it (so that it looks like it is sewn shut) and then top stitch as close to the edge as possible. You can do that by machine or by hand.
Let me know if this is confusing at all! I am typing a bit quickly, but I wanted to get back to you so you don't feel ignored. If you would like, I can try and get a video up for you!