Technologies: jQuery, JavaScript, WordPress

1. Download, unzip and add the smooth-scroll.js to your theme's JS folder.
2. Next, goto Appearance -> Editor. Open the Theme Functions (functions.php) file.
3. At the bottom of the file add the below code:
Note: Depending on the theme, you need to make sure you are inside the PHP closing tag.

function myscripts() {

    $script = get_template_directory_uri() . '/js/smooth-scroll.js';
    wp_register_script( 'smoothscroll-js', $script , '', '', true );

    //always enqueue the script after registering or nothing will happen
add_action("wp_enqueue_scripts", "myscripts");

4. Now you need to set a href link to your desired location.
5. And your destination will be an element with the same id as the link. See the Codepen example below

Follow the instructions above and instead of step 3, you will need to use a script tag in your index.html

See the Pen Smooth Scroll Example by Jeremy (@J-Roel) on CodePen.