Mostly, people use temporary license plates at the car showroom. It is to decorate the license plate on the display cars. Some people also use the plate to decorate the garage or stores. But, essentially, since it is only temporary, it is just to allow the car owner to use the car in the public but with some conditions. Some of it is a paper or sticker, so we can stick the temporary plate on a surface around the car. You can stick the plate on the car or motorcycle.
You can make the temporary license plate on a word processor application and spreadsheet application on your computer. Most people make the templates at work because they need to put the license plate on the cars. But, if it's for a small dealer, you might as well design the plate at home or anywhere you are working. Some high schoolers may make these templates at school for art class. It would be easier to make the templates on a large screen than a small screen like a phone because you need to write a lot of details about the numbers.
You can get the ideas for the temporary license plate from the numbers of the license plate that the police have. The numbers should be different from the permanent ones, so there will be no ambiguity. You can also get inspiration from a movie or TV show. They usually use a fake license plate that you can put on the temporary plate. The last thing you can get inspiration from is the internet. There are many kinds of license plates you can copy and modify.
Usually, people who sell the car will stick the temporary license plate at the showroom. They use the plate just as an accessory for the displayed car. In some countries, temporary ones are used to certifying that the car is legally bought in a trusted store. If the temporary plate is just for decoration, then, anybody can use it. But still, it is dominated by adults, especially if the number is unique and it marks the model of the car. Therefore, anyone can use the non-permanent license plate. You can use it for real car decoration or home decoration.