How to Stack

The Help Center has a good explanation. Remember that only layers with the same number of slices which are named the same can be stacked. That means you first create the individual layers, e.g., Department, IT, Vendor, then you create the stacked template. When you need to make changes to the template, make the change to the individual template layer, it will then flow through to your stacked template and your project which uses that stacked template (if you set the project setting to "Dynamic")