I bought these online for about $25.
Only if it's dark:
Test the PIR every 3 seconds
If the PIR has detected motion:
Turn on the LEDs for at least 30 seconds, continue if there is more motion
There is a handy soffit vent to bring out the PIR sensor and 12v for the LEDs. This vent is only about 12' from my Pi. I plan to run 5 22 gauge wires, 3 for the PIR (5v, ground and 3.3v signal) + 2 12v for the LEDs.
As an over-ride to my program sketched above, I have already programmed a web interface that will turn the lights on/off from my iPhone.