The important thing about customizing the theme is to not modify the core theme files. Because you will loose your customizations when you update the theme.
Some customizations can be done using CSS codes. You can add your extra CSS codes using customizer's additional CSS section.
Some other customizations need to change the template files. Do not edit the code theme files and use a child theme to override the template file. For instance if you want to make changes into the single.php file, copy the single.php file into your child theme and edit that file. If you have a single.php in your child theme, then WordPress will load yours.
Some customizations may need to edit a PHP function, put those into your child theme's functions.php file.
Note that there is already a sample child theme included in your downloaded package.