Leaving the paper release liner on the decal, position the first panel, holding it in place with small pieces of masking tape on each edge. Position the panel leaving the maximum amount of excess material to trim off. In other words, if you're covering a 2' area with a 5' piece of material, try to save the entire 3' excess.
Note: It's always best to begin applying graphics at the rear of the vehicle, moving toward the front with subsequent panels. Using this approach, panel overlap will occur from front to rear, helping to eliminate the effects of wind against the seams.