It depends on what you want the final result to look like. The Naked Neck gene is dominant so as long as they have one copy of the right gene, they will show the naked neck.
The way I read your post, you want the final result to look like a Serama except to have the naked neck. I don't know the genetics behind the coloring and pattern of the Serama or especially which color and pattern NN you started with. What I'd suggest is that you keep breeding the offspring to Seramas until you get some chicks that look like you want the final result to be, but always choose for the naked neck. Once you start getting some chicks that look like you want them to, start breeding them to each other. Eventually they should breed true.
I don't know how many genes from the original NN you are going to have to replace with the Serama genes. That's going to depend some on the genetic make-up of your original NN. All you are after from the NN is the NN gene. I would not breed back to anything other than a Serama or an offspring of the mix. Just keep repeating breeding the offspring to a Serama or mixing the best offspring until you get what you want. You may get what you want fairly soon or it may take many generations.
