Check which Apple Pencil model you have
For Apple Pencil (1st generation), remove the cap and plug it into the iPad’s Lightning port, or use the Lightning adapter and a charging cable
For Apple Pencil (2nd generation), attach it magnetically to the side of a compatible iPad
For Apple Pencil (USB-C), connect it to a USB-C cable and power source
Make sure the iPad is turned on
Wait for the Pencil to charge
Check the battery level in the iPad’s Batteries widget or when pairing/connecting the Pencil