The rule covering the golden section in art is as follows:-
The lesser is to the greater as the greater is to the whole.
It is a proportion devised by the Ancient Greeks from the harmony seen in nature (approximately 5/8).
This rule is used throughout art to give the ideal composition.
It allows for a pleasing effect on the eye and remembering it in your paintings will ensure a good basis for your subject.
Rectangular supports (canvas or board) divided in two either way should follow this rule so that your focal point is always off-centre.
Here at Localartist.org.uk we try to keep things simple and to that end the rule of the Golden Section is described best as a rule of thirds as in photography.
Divide any rectangle into nine sections. Two lines down, two lines across. The crossing point of each line provides a point, any one of which is where your main focus should be in your painting.
Balance is maintained by using the Golden Section which does not have to be measured or adhered to exactly; it is merely a guide which proves very helpful in preparing a composition.