Note the method/wrap used to string the original top on when you remove it.Use plastic wire ties also known as ty raps to temporarily center the new top.The canvas guy should give you the new string.Find the center of the new length of string and begin the install from the front center of the top and loosly string one side then the other.Then begin the final tightening making sure not to pull too much on one area or side.When tight tie the ends at the back with a knot of your choice and cut excess.Burn the ends so the string does not fray.
The guy from N.Y. did not bite?