How to do a child theme?

Answer: The best way to do this is to include the PHP_INT_MAX

add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles', PHP_INT_MAX );
function theme_enqueue_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style', get_stylesheet_directory_uri() . '/style.css', array( 'parent-style' ) );
}

kindly describe in detail about how to create a child theme
where to paste the above code???

Just read the codex, it’s really helpful
https://codex.www.ads-software.com/Child_Themes

Basically you just need to create a folder named themename-child ( replace “themename” with the actual name of the theme, then add “-child” )

Then put two files functions.php and style.css

style.css requires a header above so it can read by the theme:
Put on the first lines of your style.css Change the values as respectively.

/*
 Theme Name:   Twenty Fifteen Child
 Theme URI:    https://example.com/twenty-fifteen-child/
 Description:  Twenty Fifteen Child Theme
 Author:       John Doe
 Author URI:   https://example.com
 Template:     twentyfifteen
 Version:      1.0.0
 License:      GNU General Public License v2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
 Text Domain:  twenty-fifteen-child
*/

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The code above as you asked, should be pasted in functions.php