Callback humor is talking about something you two discussed that was either funny or interesting. If it was something funny, you’d text her about whatever it was that made you laugh. Any type of text would work. All you’re really doing is bringing up the good emotions that were released with whatever that subject was that caused the humor, thus making her recall a pleasant memory and associating that memory with you.
For a while, you bite your tongue. After all, nobody wants to be "that wife" or "that husband." You look the other way and pretend not to notice or be bothered. You force yourself to not ask who your spouse is texting and not show how worried or hurt you are. You lay awake and stare at your partner's phone, wishing you could look through it but not wanting to cross that line.
This example is just that, an example. It’s so much easier to come up with your own as this has no context for a real conversation and would be useless if you tried to duplicate it. Just try to understand the usefulness of “lol” here and there. Like anything else, don’t overdo it. Putting “lol” into every sentence, or even every few sentences, makes you look like a half-assed, not-so-funny, would-be comedian!
Ask the girl questions about herself, such as her hobbies, what sorts of music she likes, and what she did over the weekend. Try to care about what she says, even if it does not sound very interesting to you. Lastly, compliment her achievements and looks, but only if she mentions them (ie: comments on a test score or shows you a picture). If you compliment her too much, you might come off as creepy instead.