To make it look great it's important to evenly space them evenly and keep them level with the lines of the car. Believe it or not a PT Cruiser sits pretty level to the ground even though it looks like it has a California rake.
I spaced mine about 16" up from the ground and about 2 1/2" apart. To help with the spacing I cut a 1/8" piece of wood 2 1/2" wide and about two feet long.
It's important to apply these pieces to a clean surface. I used alcohol and a terry cloth to make sure that the area was clean.
Once you get the first piece on use the wood to space the other two. I made small marks with a black grease pencil to help guide me.

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.