1- First You need to register the menu, so add following code in functions.php :
Copy to Clipboard
2- Then You need add followig code to show menu in proper place in heade.php or any where you lik:
( following code also add new_menu_class to the block, so you can easily reference it by you CSS)
Copy to Clipboard
3- Next step is to add some CSS to make it horizental drop-down menu :
Copy to Clipboard