Apple's 3-in-1 wireless charging headphones can be charged by connecting the charging case to a power adapter and using the accompanying USB charging cable.
First, plug one end of the USB charging cable into the charging port of the charging case and the other end into the power adapter.
Next, plug the power adapter into the wall outlet and verify that the power supply is connected.
At this point, the indicator light on the charging case will light up, indicating that charging is underway. Wait for a while until the light turns green to indicate that the charge is complete.
Finally, you can remove the charging case, and the headset will automatically connect to the charging case to complete the charging process.