This video is spot on perfect. His method is very easy to do. Took me ten minutes to align both sides (of a Polaris Ranger 1000) and now she drives straight and does not wander. Thank you!
If back are wider than front you can still use string method by barely letting it touch the outside of the back wheels. Then the front probably won't be touching but you can measure the distance at the back and front of the front tire and adjust until the measurements to the string are the same.