The trick is to provide enough heat to stimulate the seed into germination but not enough to kill it. This treatment is designed especially for seed of those species that live in areas with fairly regular forest fires. The seed does not germinate in the wild until after being stimulated by the heat of a fire. This can be of two kinds, warm or, more usually cold. Stratification is a way of convincing the seed that it has passed a winter or a summer and winter and that it should now germinate.
Cold stratification involves keeping the seed in a cold place for a specified period.
The salad compartment of a refrigerator or a shady north wall outdoors in winter can both work. A freezer is not recommended. The temperature for cold stratification is usually in the range of 2 to 5c, it should not normally be frozen. Warm stratification involves putting the seed in a warm place, perhaps an airing cupboard or a warm greenhouse for the time specified. It must be stressed that the seed has to be moistened before stratification and must not be allowed to dry out.