To make it look great it's important to make it match perfectly with the indent that goes around the car. In talking with Mitch at Cruiser Motor sports it was recommended that the beltline be mounted just about the indent and not in it. Apparently moisture can get trapped in there and over time cause the adhesive to release.
To make a perfectly straight line I used a 3/8" wood dowel and held it in the groove while I placed the chrome strip up against it.

I like to start out with by peeling about four inches of backing tape off and start placing the piece then peel the rest of the backing off as you proceed.
It's also a good idea to do this in temperatures above 60 degrees F. If it's not that warm where you are working then you can use a hair dryer or heat gun to warm the area.