I've read other string methods and there are several variations. Heres the way i intend to try to do it next time i attempt an alignment.
Put a string around the back of the rear wheel. probably wrap it thru the wheel a few times to hold it in place well. Take the string on each side of the tire and tie the end to a heavy weight like a cinderblock. Each side the string should be long enough to pass the front wheel completely. Now bring the weights past the front wheel and move them into position where the string just touches the rear tire at the front of it. So the string is coming off the rear of the rear tire then touching the same part of the tire at the front of it with the weight holding the string in place beyond the front tire. Now turn the front wheel till the string is the exact same distance to the front edge of the front tire and the rear edge of it. The measure the opposite side and if they fornt and back edges are the same measurement it's aligned. If not then you know what to do.
That to me seems like the easiest way to do it and the simplest to grasp. Tho my description may be hard to follow i tried to make it as clear as i could.