If you're using Photoshop, create the license plate FLAT, then use
edit > transform > distort to "distort" the rectangle (plate) to the plate of the car.
move the 4 corner points to the 4 corner points of the existing plate.
You could use layer style > bevel and emboss to give the text a raised effect like a real license plate.
Also, eye-dropper the original plate-text to see what colour it is. So the your text on your plate, is the same colour. Pure white will stand out too much; doesn't look natural.