When applying vinyl keep the lettering running level at all times (unless of course the design is deliberately angled). Apply the application tape level with the bottom edge of the lettering. Once you have determined the rough positions of each set of lettering on the vehicle use a soft pencil to mark the position. Then use a tape measure to put a series of marks on the van to ensure the lettering runs straight.
If this is your first attempt I would suggest you apply the vinyl wet. Make up a weak soap solution (water and washing up liquid) to wet the surface of the van and the sticky side of the vinyl. Applying wet allows you to slide the lettering into position and also means you are less likely to end up with lots of bubbles of air trapped behind the vinyl. You have to wait a while before you can remove the application tape so the whole process takes longer than applying dry.
Good luck 😀