Turn an existing page into a modal

I want to convert an exisiting Login page (built with another plugin) to a modal. Details:

I have already created a functional login *page* using another plugin (Ultimate Members), and have then styled the page just the way I want it. See aa.afhs.ab.ca/login. This login page was created by a built-in login form in the Ultimate Members plugin. The page uses shortcode to call the login form, result on the front end is the page aa.afhs.ab.ca/login

I want to use the Modal Popup Box plugin to “convert” this page into a modal. What I mean is that when someone clicks Login on the main navigation menu, the click results in a login modal, instead of the current situation where the login “form” is actually a page.

I’ve tried a solution via shortcode, the best I can get is a modal inside my exisiting login page. Well, I don’t want that. I just want a modal INSTEAD of the login page.

How, specifically, can I do so? Can someone step me through it?

The page I need help with: [log in to see the link]