STEPS:

Connect the Windows PC keyboard to the Mac as usual, either by USB or Bluetooth

Pull down the  Apple menu and choose “System Preferences”

Click on “Keyboard”

Choose the “Keyboard” tab and then click on the “Modifier Keys” button in the lower right corner of the preference panel

Choose the PC keyboard from the “Select Keyboard” dropdown menu at the top of the Modifier keys screen to insure you are modifying the proper keyboard connected to the Mac

Click the dropdown next to “OPTION Key” and select “Command”

Click the dropdown next to “COMMAND Key” and select “Option”