Divide the artwork into two equal halves. With simple proportion, the artwork will be almost exactly the same on both halves. This is usually more common with three-quarter portraits. The example artwork is an example of simple proportion.
More complex proportion involves balance on each side of the artwork. This is accomplished by having items of the same shape, size or weight on both halves of the artwork. The example artwork accomplishes this concept because the child on the left balances with the child on the right.
Here is another example of balance creating proportion. The bulk of the black couch balances the length of the female's form.