It depends on weather you think you are getting too much fuel to the engine or not enough. Smell is one way to tell, or look below to see if fuel is pooling below the engine. If too much...leave the mixture out, crank and slowly push the throttle in about 1/4 - 1 inch. When it catches, push in the mixture. Not enough fuel, prime more (3-4 depending how cold). I've started 172's down to about -45F. When in doubt though, preheat. At a minimum it will be better on the engine. Probably the most important part is SLOWLY advancing the throttle until you find the sweet spot where it wants to run. Not enough is no good, just as too much is no good. Just like the three bears....it has to be just right.