Like everyone said above, Acme printed on photo paper is easy.
The USA plate was made in Photoshop and printed on decal paper. The plate is made from an aluminum soda can with Grandtline bolt heads. The plate itself was from http://www.worldlicenseplates.com/ which has military plates and plates from all over the world. I did have to add my plate number in Photoshop. The soda can aluminum was more realistic than anything else.