Please... More hints to add sub-menus to BlogsMU

Hello:
I think I'm in a place where I need to add sub-menus to BlogsMU to conform with the design of growing pile of sites & subsites.
Here are the bytes I've found so far:
* Code goes in navigation.php (https://premium.wpmudev.org/forums/topic/few-questions-1#post-39011)
* There is a function named register_nav_menus() used by BuddyPress, which appears to do the trick for most blogs... wp-includes/nav-menus.php

I'm not seasoned in php - so any advice about how and where to call this function to enable menus would be awesome helpful in getting me started.

The string "menu" does not appear anywhere in navigation.php, but the last line calls: <?php do_action( 'bp_nav_items' ); ?>

I'll happily post my mods here to help answer the other several questions on the topic.