I've seen videos of people customizing their cars and they put their plate on in the LS Customs, however when I tried it, I was unable to find an option for that. I downloaded the app and tried to do it from there, but I was only able to do it to one vehicle, and there is no option to change vehicle. Is there a better way to do this? Am I missing something?
Once you have navigated to the garage tab under Los Santos customs you will need to scroll past the three main character's cars. At the bottom of the tab you should see your Rockstar Social Club account. Any cars that you currently have stored in your multiplayer garage will show up here. You can swipe left or right to navigate through the cars that are currently stored in your garage.
I looked into this a little further, and some people have encountered a bug where it will only show them the latest car they stored in their garage (I do not have this bug, I am able to swipe left/right to select my car). So you might possibly try driving the car you want to edit out of your garage, and then driving it back in again. Annoying and time-consuming, but it seems to work for other players that have encountered this bug.