when you buy the decales find out if they are wet applied or dry applied.
Wet means that you can use a solution of water and a few drops of baby shampoo and get the CLEAN surface wet and then put the decale on o it . this will allow you to move the decale around to get it in the right place.
once you have it close to where you want it start in the middel and work to the out side to get the bubbels out of it. then let dry and get as much water out as you can.
Also on doors and hood seams. you need to cu tthem in the center of the gap and then roll both edges to each side and around the back as far as it can.
then after this follow the directions on clean and care of them and how long it will need before you can wash it etc...
hope this helps