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// add to functions.php

function register_my_menus() {
  register_nav_menus(
    array(
      'header-menu' => __( 'Header Menu' ),
      'extra-menu' => __( 'Extra Menu' )
    )
  );
}
add_action( 'init', 'register_my_menus' );


// add to theme

<?php wp_nav_menu( array( 'theme_location' => 'header-menu' ) ); ?>