I bought pre-made N-numbers (vinyl) from A/C Spruce. In photo 1 there is painter's tape holding the top edge of the sheet with the numbers. To apply, lift up the bottom of the sheet, peel off the backing material, then use a squeegee from top to bottom to apply the numbers smoothly.
Photos 2 and 3 show the happy results!
There were a few small bubbles in the numbers, but a careful poke with an Olfa knife, and another smoothing with the squeegee, got rid of those.