You can go about this in a number of ways.
1) Copy and paste the script tag containing your font-awesome kit in your footer.php before the closing body tag.
2) Using wp_enqueue_style. Below is a working example of a custom WordPress theme with FA enqueued:
`function theme_enqueue_scripts() {
wp_enqueue_style( ‘Font_Awesome’, ‘https://use.fontawesome.com/releases/v5.6.1/css/all.css‘ );
wp_enqueue_style( ‘Bootstrap_css’, get_template_directory_uri() . ‘/css/bootstrap.min.css’ );
wp_enqueue_style( ‘Style’, get_template_directory_uri() . ‘/style.css’ );
wp_enqueue_script( 'jQuery', get_template_directory_uri() . '/js/jquery.min.js', array(), '3.4.1', true );
wp_enqueue_script( 'Tether', get_template_directory_uri() . '/js/popper.min.js', array(), '1.0.0', true );
wp_enqueue_script( 'Bootstrap', get_template_directory_uri() . '/js/bootstrap.min.js', array(), '1.0.0', true );
}
add_action( ‘wp_enqueue_scripts’, ‘theme_enqueue_scripts’ );`
\
You can simply copy and paste the code(within the back-ticks) in your functions.php file, then modify to suite your need. Hope that helps.