Next youll want to set up your entire navigational layout - any buttons, input text areas, additional interfaces, and displays that you think youll need. It doesnt matter if its just a symbol that doesnt do anything, or even a graphical placeholder; just try to arrange it as best you can and leave yourself room to expand as you go. It doesnt all have to be on the same layer, but it does need to be grouped inside a single symbol (it doesnt matter if there are other symbols inside the main layer).
The reason for this is because every time we change the skin, well need to change the depth of the entire interface so that it stays on top of the base skin layer. Its easier to do that all as one symbol instead of having to change the depth of every last little bitlet individually.