Here's what I do. With the decal still on its paper backing, position it on the body and tape the edges down. When you are sure of where it's supposed to go, make some very light pencil marks on the body where the tape will actually sit. Also make some registration marks to help like the stripe back up.
Now feel the paint for hickies, dust, bump, anything on the paint that might form a peak underneath the decal. If you find anything in the paint that will prevent from laying flat, wet sand that area with 800 grit paper, or finer, til it is smooth. Wash the whole area clean when done. I like to use prep sol on the whole area to remove any wax.
Mix a little detergent in some water and wet the are to apply the decal. If you need hints to help position it, look at other members cars first. If its the tail stripe I think it is, like a 70 Dart, there is a natural point or crease where it goes over the top of the fender. Some guys lay the decal on like factory, where it abruptly ends where the rear lower valence starts, other guys continue it down the rear valence. Your choice. It sits almost at the very edge of rear qtr where the qtr extension and the qtr meet, and on a 70 about in the middle of the side marker lite.
And the point in the middle of the big decal on the trunk follows the V shape towards the back of the car, same as the trunk lid itself.
Use plenty of lightly soapy water and when you are sure its down where you want it, use a squeegee or clean bondo spreader to push out the water to stick it down where you want it.