What year and model do you have?  It could be that you're already pre-wired.

If not, the next question is how legal do you want to be?  Legal means
wiring them so that they can only come on when low beams are on.  Not
quite so legal means otherwise.

If legal:  Connect the coil of a relay to low beam, and put a switch
somewhere in that same circuit.  The order of what comes where doesn't
matter, just as long as the relay coil gets its electricity from low
beam, there's a switch somewhere in the circuit, and the other side of
the coil ultimately gets to ground.  Relays are common items at the auto
part store, and quite inexpensive.  Look for one with 4 pins and a
wiring diagrem that shows which pins go to the coil and which to the
contacts. (I am assuming that your car is of the variety that the
headlights go off when the ignition is off.  If not, then you have to
take a more advanced approach, possibly involving two relays, or take
the chance that you'd leave all your lights on by mistake.)

If other than legal:  Power the relay from some place that's on only
when the ignition is on, such as perhaps the lighter.

Then buy an in-line fuse holder which has the appropriate guage wire for
your lights.  10 gauge should be safe.  Connect one side to the battery
positive, insert a fuse of the right amperage, connect the other side to
one contact of the relay, connect the other relay contact to your lights.