Some of how this is handled may depend on what work is popups do. If their actions are pretty much independent of the pages they’re on, it should be pretty straightforward.
It would be helpful if you describe what you’ve done so far and where you’re stuck.
The reusable element will be in the reusable elements section at the bottom of the elements list in the UI Builder. Just paint it anywhere on the page.
On the page, it will get its own name that may be different than its original name. For example, a reusable element named reDoSomething may have the name reDoSomething A on the page. Like any other element, you can change its name on any page where it’s used.
If the element has a Type of content and the page must provide it, use the Display data action to put that into the element.
To show the element, use the Show action. You’ve probably already done this before making the popup reusable.
In every case where I’m using a reusable popup, the popup hides itself with the Hide action, naming itself.
Now, I suspect this may not have answered your question, because this is all pretty much the same as when your popup is unique to a page.
Let us know where you’re stuck and we can guide you. Pictures may be helpful.