Well, there isn't much you have to do - the warming water and approaching spring will do it all for you. If the fish have been fed properly before the cooler weather approached, and are old enough, they'll spawn on their own. I heard from various breeders that 68 degrees is the perfect temperature for spawning and raising fish, so let nature's warmer weather do its thing.
Outside fish have a much finer sense of weather change and such than indoor fish, and will do what comes naturally to them. If you have nothing in their tank right now, try to get some artificial plants, or even a tangled up piece of yarn, so the eggs have something to attach to. That, and the fish also like to chase each other into plant matter, it helps expell the female's eggs.