Before mounting the propellers in the correct manner, we need to test each one and determine if they need further balancing. Balancing the propellers is important and an unbalanced propeller results in "shaky" hovering of the quadcopter. To minimize the shaking, you have to constantly fix each drag with the sticks on your remote controller and this is quite annoying; especially when the goal is to capture a clear photograph.
There are tools that can be used to balance your propellers easily, as shown in the following image:
As you can see, the left propeller is perfectly balanced and the right one is not. But how can we fix a badly balanced propeller? We can simply put some low weight on one side of the propeller. For example, you can use Scotch tape. The weight of a 1x2 cm piece is enough to change the balance. Place the tape on the side that is higher and you will see that the extra weight that we have placed balances the propeller perfectly...