Every different boomerang style has its own specific flight characteristics due to its aerodynamics, wood density, and weight balancing built into the 'rang. There are too many variables of design for weighting rules to be iron-clad so don't be afraid to experiment. Sometimes combined ideas produce the desired effect. Weighting can be used to adjust the flight of a particular boomerang to your particular throwing style. Once you are getting consistently accurate returns - begin moving a weight, or weights, around the boomerang to see what the results are.
Some people like to weight every rang they own on the lift arm near the elbow.
Things used for weighting are coins, lead tape or flattened lead pieces of different size. They can be attached with electricians tape or colored plastic tape available at hardware stores in the hobby section. Attaching the weights to the bottom of the wing decreases the lift which is desirable in a strong wind. But weights attached to the top of the wing work very well, too. Experiment to find out which method produces the desired results.