Why use a child theme?
Child themes are the recommended way of modifying an existing theme. For example, if you modify the original theme directly you may lost all your work after theme update. That’s why better use a child theme for customizations. In this case you can be sure you won’t lose your progress. Read more here https://codex.wordpress.org/Child_Themes.
How to use a Child Theme?
Just install and activate as a normal WordPress theme. You can add all modifications to style.css or functions.php in your child theme folder. You can find a child theme inside the Full Theme Package on the ThemeForest.