Send a text within 24 hours and reference where you met. Keep it simple and casual, anchor it to something you both know, and don't overthink it. Both of you are nervous anyway.
So assuming you are texting a girl who is not a drama queen, you want to do it within the first 24 hours of getting her number. Again, just keep it simple. "Hey, it was really fun to meet you at such-and-such a place," or "I was so glad that this person introduced us," whatever it is. If you add a little bit of context of whatever was going on when you met her and got her number, what this does is it helps keep the conversation going. Now you have something else to to talk about, the location or the person who introduced you or whatever was going on at the time. And here's an important thing to remember with this and all the tips I'm going to be sharing with you. Don't overthink it. We tend to overthink it, right? It's really, really easy because you want to make a good impression and that's totally normal. The good news is you can and should just keep it casual and really natural feeling because— yes, she's nervous too. Okay? So if you're feeling nervous, she is too.
