Adding a sub drop down menu.

Hi

I would like to add some pages that are sub to the main menu and as a result have a drop down menu linking to the sub pages:

i.e https://wpremix.com/demo/

I see that there are various plugins for this but can you do this yourself as I already have a horizontal menu and just want to add to it.

maybe something along the lines of:

<ul id=”dropmenu”>
<?php wp_list_pages

any idea ?

Regards

Conrad