Prep wise you really need to clean and de-grease the tires....especially if not new. I then used acetone and a green brillo pad to remove any hidden
oil or grime. I then followed the the instructions by using alcohol to clean the application area and rear of the letter prior to putting on the glue before gluing it down. I recommend using a heat gun or hair dryer to evaporate the alcohol on the letter and tire completely.
I placed them on the tire and once happy with the alignment and location, I used chalk to mark corners so once I put glue on the letter I could replace it exactly where it was. I also recommend using clear tape across the front of the letter to help place it on the tire. That way you aren't touching the glue on the letter and make a mess and it's easy to see when placing it on the tire its lined up like you want. I also recommend doing it with the tires flat on the ground... Not on the car.