1. Unzip it
  2. Rename the folder according to the name your the parent theme (example: for Twenty Fifteen -> twentyfifteen-child)
  3. Open style.css and add the parent theme name where it is indicated
  4. Upload the directory into /wp-content/themes/HERE
  5. In WordPress, go to “Appearance ->Themes” and activate your child theme

In style.css, you can replace my name and website by yours if you wish.