Choose a popup plugin that have free features to create popup templates, analytic tools to measure visitors’ interests, easy integration options with big forms and email client service providers, and it should be responsive to work equally on all screens and browsers. It shouldn’t affect page load speed.
Create beautiful and product relevant templates that may attract the visitors, and soon you will get fruitful results. Make the email list according to user interest and don’t send them irrelevant materials that may cause unsubscription.