My graphics dude told me to put mine on dry - dunno if that is just the material he uses.
Thoroughly clean the plastics with isopropyl alcohol. If you have old stuff on there, Goof Off to remove the old chud.
Then, what I did was remove a half inch slit from the middle of the backing. So, on the front number plate, was a half to 3/4" vertical slit, I made with a razor and peeled off the backing in the middle.
Located it where it needed to go, stuck it on gently, and when I was sure it was right, pressed down to stick it firmly to the plastic. Then removed some backing on one side, and gently worked it on, making sure the bubbles were worked out. Took my time and did it slowly. Peel a little, work it on, peel, work it on all the way across slowly until it was on, then repeated for the other side. He told me to do this somewhere warm, and leave it somewhere warm to settle for a day or so and if you have issues getting it to conform, a hair dryer or heat gun works wonders.
this may not apply for the material yours is done with, he said with the old stuff he used the wet technique and it was better but for this new 3M backed stuff to do it dry. It is pretty thick stuff and conforms to curves well. I have zero bubbles on 2 sets of graphics, (number plates on the KX, and full graphics on the pit bike)
So, your mileage may vary but this is what worked for me.