For example:
You animate the head in three states.
Head neutral. Head smile. Head hurt.
Based on the animation, you will use one of those three heads.
Then you have an "orc" skin.
So you would make the images accordingly. Orc head neutral, Orc head smile, Orc head hurt.
Then create 3 skin placeholders. Head neutral. Head smile. Head hurt.
Put Orc head neutral in Head neutral.
Put Orc head smile in Head smile.
Put Orc head hurt in Head hurt.
If your problem is that you need to combine different head types with different armor types, that's not something you can currently preview in the editor.
But what you would do is have the different head types as skins, and different armor types as skins. You would not put the head types and armor types in the same skin unless you want them to be inflexible.
At runtime (on the game engine side), you would create a new skin by combining the skins you created in Spine.