You just need to bake it longer than specified in the recipe; baking time varies depending on the oven. About the yolks, beat it until it turns pale or cream in color, and very light; it’ll probably take about 5 minutes, it depends whether you’re using a stand mixer or handheld. For the cream of tartar, you add it with the egg whites, before beating; it acts as stabilizer so the egg whites will keep it’s form but no need to buy cream of tartar if you don’t have it, it’s not really necessary, just make sure that you have clean, oil-free bowl and whisk and that your eggs are at room temperature.

P.S Try cooking it over stove top too, it’s easier and faster :)