Some people don't balance props in the 40 size engine range and only on bigger stuff. I balance all mine, it helps reduce vibration and an unbalanced prop can actually cause alot of vibration.
To balance a prop you either add weight to the light side or remove weight from the heavy side or a combination of the two.
Remove weight by sanding lightly on the back side of the prop.
Add weight by painting the tips of the prop.
There are other methods and others will chime in.