How to save Setting pages data to the database?

I have created a setting page and I have a basic form in there I want to save its data in the database how can I achieve that. This is the HTML form:

label class=?php _e('Message', 'domain'); ? /label
textarea class=message form-control name=my_settings_fields[message]?php echo esc_html($my_setting_data[ 'message' ]); ?/textarea

This is how I registered my setting page:

  public function my_register_settings()
  {
    register_setting('my_setting_option_page', 'my_settings_fields', 'sd_callback_function');
  }

  /**
   * Register the plugin settings page.
   *
   * @since    1.0.0
   */
  public function my_register_menu_page()
  {
    add_submenu_page(
        'edit.php?post_type=post',
        __('FAQPress Settings', 'domain'),
        __('Settings', 'domain'),
        'manage_options', 'my_settings',
        array($this, 'my_add_setting_page'));
  }

  /**
   * Callback for plugin setting page registration.
   *
   * @since    1.0.0
   */
  public function my_add_setting_page()
  {
    require plugin_dir_path(__FILE__) . 'partials/my-admin-settings.php';
  }