Use their site to put in the plate what you want (after you choose the state or province you want) then just right click and save as a jpeg. And Bob's your uncle.
Made this one in about 10secs.
You then just have to print it on some decal paper, and away you go.
He has more plates, but they are just blank, so you have to do some editing in photoshop, or similar program to put the letters on it, more choices, but a little more involved.