What to do, if the flyer is nog well-balanced and you have vibration when spinning?

The non-balanced flyer makes a lot of vibration and noise by high speed spinning (15:1 or higher). I myself find it very irritating. Moreover it causes premature wear of details of you spinning wheel.

The original method is published on site of Ashford. I just improved it for more precise balancing.
(English, with Russian subtitles):

Moreover, it should be borne in mind that in addition to the weight, in this case it is also important torque: depending on the position of the load on the flyer arm, it will be different. Therefore, after the load is found, it would be good to try to put it in different places of flyer arm, in order to choose the optimum position.

The “suitable” screw you have to screw in you flyer. But be carefully, if the screw is very big, the flyer could be broken. So, in that case, drill the first a little hole in your flyer, and repeat balancing procedure again, because after drilling you take some amount of wood out you flyer, and this means, it need new measurements.