Create New Mobile Menu

Hello,

I want to create a new mobile menu for: https://www.totallypurrfect.com so that the desktop version stays the same but on the mobile version - there's category drop downs below the "blog" page.

How do I do this please?

I found these instructions: https://docs.generatepress.com/article/mobile-navigation/ but I'm not sure they're what I need for creating the mobile menu from scratch as it speaks about setting the width and I can't see anywhere to do that on here: https://ibb.co/TbfqTwy

Can you please advise as to how I make it?

Thanks!
Ella