Dumb Question, but how on earth do I start a conversation over text?

I was at a gathering recently and started flirting with this girl, we seemed to get on really well and we were just about to exchange numbers when my ride got impatient about leaving.

Recently, I got my friend to give me their number… but have actually no idea what to say. It’s been a week since this gathering happened, so there’s none of the easy segues I was hoping for.

Largely, I just need a first message idea that doesn’t come across either too nervous or too strong.

I have no idea where might be the best place to post this so chose here, help 😭