How to wire a timer switch?

I am wiring a timer switch to my outside GFI for Christmas lights. From the wall I have 2 black wires which were connected to the existing light switch. There are 2 white wires, both of which were capped off and not connected to anything. There is a copper ground wire as well. My timer switch comes with a... show more I am wiring a timer switch to my outside GFI for Christmas lights. From the wall I have 2 black wires which were connected to the existing light switch. There are 2 white wires, both of which were capped off and not connected to anything. There is a copper ground wire as well. My timer switch comes with a black, red, white and green. No matter how I connect it, the lights work, but the timer does not turn. Please advise? Thank you.
Update: Thanks Chris, but that's the way I have it wired already. The lights work fine with the on/off switch on the timer, but the circular timer does not turn in order to activate/deactivate the lights. Maybe I've got a defective timer.
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