The speaker is located on the front right side of the Model 3 (S & X are probably the same, but at the time of writing, we can't confirm!), so when parked, turn the wheel as far is it'll go to the LEFT, thus providing easy access to the front right wheel well/liner.
Grab a medium flat-head screwdriver, and remove the 3 'pop-pins' nearest you, that hold the wheel well liner to the body.
The liner is flexible, so once the pop-pins are removed, you can pull it back to reveal the speaker.
Remove the plug from the speaker by pulling back on the grey retaining clip, and then pull the entire plug out.
Now take the female plug from the kit and use it to seal the cable. Use 2 of the blue seals to weather-proof the back of the female plug. To prevent the cable being lose, you might want to take some electrical tape, or a cable tie, and secure the cable to itself.
Now take the male plug from the kit, and plug it into the socket on the speaker. Again, use 2 of the blue seals to weather-proof it.
Now replace the 3 pop-pins and secure the wheel well liner. You're done, enjoy the silence.