How do I prepare my InDesign package for the Template Studio?

Get all static content in one layer and lock it. Set up a layer per variable issue, and tag it.

Start by assembling all static content and put in one or just a couple of layers and lock them.  Now select the first variable in the design and tag it. You can find the Tag tool under "Windows" > "Utilities" > "Tags". Or see the screenshot below:

indesign tags window

In "Tags" you can create the wanted Tags for text and images. Make sure that every variable (text or image) has its own tag.

indesign tags window2

Now save the design and create a package including all the Links, Document fonts, a .idml file, and a PDF file.