MixAndMatch.cs is a sample script. It wasn't designed to be used with anything but the sample assets.
As with the other sample scripts, they're designed to be read so you understand what code needs to be used to achieve their effect, and use them in your own scripts. This is why it's in the Examples folder.

Here's how the sample MixAndMatch.cs script works in short:
You give it the name of your base/template skin as an information source, where it can find the template attachments.
You give it the slot and names of those template attachments you intend to use.
You give it sprites to use as the new images for those templates.

then
It uses the specific sprites, and the template attachments for them, and puts them in a new custom skin.
It optionally repacks all the sprites into a runtime atlas. If it does this, it also includes the default skin (the stuff that's not in skin placeholders in the editor) in the packing.
Then it gives the new custom skin to the Skeleton as its new active skin.

We're open to suggestions and feedback if you find that the examples aren't clear enough.