Hi Jeon,
The grounding kit is applied to the dampers and the sustain Damper rail. If you are not replacing all of your damper arms in the piano, then what you will do is put the conductive foil tape across the top of the damper arms next to the fastening screw. This creates a shield and grounds all dampers together. Next, you will take the black wire with the solder lugs and go from the end damper arm to the screws on the damper rail that hold the plates over the damper rail pin.

It is advisable but not necessary to replace all damper arms and use the foil along the wooden action rail underneath the new arms. New damper arms offer a new life to the piano action.