Here is an extract from an Arthur P post which is just about the best explaination on walkback tuning that I have heard. It would be assumed that the bow is shot exactly perpendicular to the gound (no cant) and the site pins are exactly parellel to the string. This method however will only be as good as theindividual shooting the bow.
walkback tune to clean up your centershot. So, start over from scratch. Eyeball centershot where you think it needs to be. Move the SIGHT to sight in dead nuts at 20 yards, then move over to the 50 and fire for effect. Move the CENTERSHOT so that the arrows are flying down the middle at 50, then go back to 20. Sight in dead nuts at 20 again, then move back to 50 and fire for effect. Adjust centershot to put the center the arrows on the pin again. Then go back to the 20 and repeat the whole process again. Only move the sight at 20 and only move centershot at 50. Shouldn't take more than 3-4 rounds of that to get your centershot and pins wiggled in and lined up properly.
This is the way I always tune centershot on my bows. Since I know I'm going to walkback tune anyway, I never paper tune any more.