Summarize:

Hold your decal up against the surface you want it on. Move it around to see which positioning you like best. If you want the decal to be centered, measure the surface from top to bottom and side to side with a tape measure and mark the center. Use a level to make sure the decal is straight. Lightly draw a line along the surface to mark where the top edge of your decal will go. If you're working on a window or the outside of a vehicle, use a strip of tape to mark the surface instead. The top edge is the edge at the top of your decal. Don't remove the top lining or the backing on the decal yet. Line the decal up with the marks you made and apply the tape to the top edge of the decal. Use a long, continuous piece of tape to cover the entire top edge. If you're putting up your decal alone, tear off a piece of masking tape before you hold the decal against the surface so you're not struggling to do it with one hand.
Choose where your decal will go on the surface. Mark where your decal will go with a pencil. Tape the top edge of your decal to the surface with masking tape.