There are a couple of steps.
1. Take your photos from as perpendicular a position as possible.
2. Take a reference measurement and write it down for each shot. The bigger the better. I usually measure from the door to the taillight on the sides and from taillight to taillight on the rear.
3. Bring photo into PhotoShop or Corel PhotoPaint and rotate it to a level position. Photoshop has a measuring tool that captures the angle once taken and you then execute a Rotate Canvas command using the Arbitrary setting. Save the photo with a new name.
4. Bring the new photo into your signmaking / design software. Draw a box where you took your reference measurement. Select the box and enter your reference measurement making note of the percentage of the resizing. Cancel the size change. Select All and do a proportional resize by percentage using the figure you just noted.
You now have the vehicle in your
sign making software at full size and level. You should be accurate with this method to within 1/2" overall.