Spoiler warning: we wont. Overlays are inherently terrible.
Making a successful overlay #1 - the timing
The overlay needs to appear before your victim leaves the site, so it needs to be quick. Never mind that "quick" roughly corresponds with the amount of time someone who is genuinely engaging with your site takes to orientate themselves and settle in to proper reading. In order to catch those people leaving quickly because they don't care anyway you're going to have to annoy your real visitors. Omelettes, eggs and all that.
Making a successful overlay #2 - make it intrusive
You're trying to catch the attention of your victim and what is more attention-grabbing than the entire screen being replaced by the action you want them to perform? Maybe it should flash too. Make sure they are under no illusions - your only interest in them is harvesting a review or an email address. Any content on the site is strictly secondary.
Making a successful overlay #3 - make it alien and hard to close
Making a successful overlay #4 - make sure it is in the wrong place in your visitor's journey
You can't possibly wait until your victim has finished reading the page before asking their opinion or for them to sign up. They might leave at the end, having learned whatever they wanted, possibly even happy - and all without ever giving you their email address. The horror. Never mind that five seconds into a visit nobody has a useful comment beyond "the overlays annoy me" and that putting them right at the start of the visit means your overlay cannot possibly do the job it is supposed to do. Annoyance is valuable feedback so it should be gathered as early as possible.
Make sure you're considering these crucial four points next time you want to really annoy the poor souls who find your site!