Those modal overlays/popups are being used a lot on mobile either timed after an article or triggered by certain actions. What are best practices for using them best for lead capture forms?
Keep in mind what your users' purpose is. They're not likely to appreciate being interrupted from doing what they came there to do (e.g., reading your article, browsing your catalog, checking out).
I'd suggest that you avoid popups and modals that interrupt like that. Instead, place one or two calls to action in strategic locations on the page. True, users can ignore these, but when you take away that freedom, lots of users get frustrated. I know I do.
(The real answer to these "What are the best practices for..." questions comes from competitive research. Look at other sites and see how they do this.)
The best practice is to test test test.
Your site is not identical to any other site, nor does it have the same demographic. You find out what your users respond to by developing hypotheses and then testing and evaluating them.
Also there's a million articles about this and A-B testing, so go look them up.