Turn on the wireless keyboard and mouse using the silver button on them until shows Green.
Take the USB-C cord and plug into the wireless mouse & back of the iMac to sync. Then unplug the USB-C cord from wireless mouse. Move the mouse to make sure it works.
Next take the USB-C cord that should still be in the iMac and plug into the back of the wireless keyboard to sync. Type on the keyboard to make sure it works.
Then unplug the USB-C cord from the wireless keyboard and iMac.
