So when I made my plate I didn't read the fine print and now only my bifta has a custom plate. How do I add it to my other cars?
You should just be able to go into any Los Santos Customs in the car you want the plate on, go to plates, pick the colour/style you like and then select the option at the bottom of the list to load in your custom plate text.
Alternatively you can do it on the iFruit app. Go into the Los Santos Customs section, select the plate in the top right that you want, select the car you want it on, then just press ORDER. The confirmation should then just say you are ordering your custom plate and that it is free (if you already own that style)