Most important detail about showing a popup: Make it unique.
In this example from Rothy’s, they’ve made 2 clever choices:
The shape of the popup and close button location.
You might think “how can close button location possibly make a difference?”.
It does, and here’s why: users expect to see the close button at the top right corner. The bottom right corner is unconventional. That means it takes the mind a fraction more time to locate it. In a world were consumers are making snap decisions thousands of times a day this minute switch could make a small difference. And small differences add up.
The unconventional shape works for the same reason: users don’t expect to see a circular popup so they slow down to process it (like you’d slow down if you saw a green elephant on a Safari). As a result, more people end up reading Rothy’s email signup pitch. More readers = more signups.