Hello tuhai993,
So you said you read the documentation here: Texture Packing - Spine User Guide
As it says:
While it can be used for your Spine skeletons, it can also be run separately to pack all the images for your applications.
That's because what it does is taking all the images in a folder and pack them together. It outputs 1 or more png file and an atlas file that maps the position of all the images.
I think the guide is pretty comprehensive, you can pack what you need and then parse the atlas file in your preferred game engine to use those images. Skeleton files are not generated because the purpose of the utility is to pack textures, still, this is pretty useful because you can pack all of your game assets together.
If you don't need to have everything packed together but you just need a single atlas file you can simply generate it by exporting a json or skel file and select the option to generate an atlas, which is very similar to the options that texture packer provides.
You're right though, a tutorial would be great, I'll take note to make one (: thank you for the suggestion!