Installing a Storefront child theme is crazy easy and is no different than installing a regular WordPress theme.
The only requirement when installing a Storefront Child Theme is that the parent theme Storefront is already installed.
So first things first make sure you have WordPress, WooCommerce, and Storefront installed. You can see how to do that here.
Next find the zip file of your Storefront Child Theme and go to: Appearance > Themes > Add New then click upload.
Activate the new child theme and boom, you’re done.