Allow egg whites to come to room temperature for 30 minutes.
Combine egg whites and cream of tartar in a clean, dry metal or glass bowl. Beat at medium speed until egg whites form bubbles and look frothy.
Add the sugar, one tablespoon at a time. Beat on medium speed between each addition until the sugar is well incorporated. At this point, when you pull your mixer out of the meringue, the point that forms will fall over on top of itself. This is soft peaks.
Keep beating the eggs whites until they become more dense and shiny. You know your meringue is ready when you pull the mixer out and the point stands up straight. This is stiff peaks.
Spread or pipe the meringue over the top of your pie as soon as it comes out of the oven. Spread meringue all the way to the edge where the crust and the pie meet to seal the meringue.
Return the pie back to the oven. Bake until the meringue begins to brown, about 10-15 minutes.
Allow the pie to cool at room temperature.