To get yourself oriented, see this video Starting at the back of the bowl, a rubber spatula is used to cut down vertically through the two mixtures, across the bottom of the bowl, and up the nearest side.
The bowl is then rotated a quarter turn with each stroke, continuing the down–across–up–and–over motion.
This gently turns the mixtures over on top of each other, combining them in the process. A rubber spatula is the ideal tool for this method.
Always fold gently, but thoroughly, since under-folding can leave pockets of foam that are not combined with the batter, which then forms a sticky wet layer on the bottom of the pan after baking; over-folding can result in a product that does not rise appropriately in the oven.
If the batter is very thick or heavy, first stir in about a quarter of the lighter foam. This will loosen the mixture, and enable the remainder of the foam to be folded in with ease.
Generally, the folding process should take about a minute, to avoid over folding. Some streaks may remain, but these should be thin and even. If the batter is evenly streaked, it is generally okay to stop.