Based on the iPad model, refer to these diagrammed instructions to determine the SIM tray location.
Utilize the SIM eject tool (or paperclip) to unlock the tray by inserting it into the provided slot.
Insert the SIM into the SIM card tray.
To insert the tray and SIM back into the iPad, align in slot and press in on the tray to lock it into place.
With the iPad screen facing up, the SIM card tray is inserted with the SIM ID on top and gold contacts facing down.