In a clean mixing bowl with a whisk attachment, beat the egg whites on medium low speed until foamy. (I set it to speed 2 or 4 on my KitchenAid stand mixer.)
When egg whites are foamy, add cream of tartar and salt and continue to whisk.
Then slowly add sugar one tablespoon at a time, while mixer is still running. Allow the sugar to dissolve after each addition.
Continue beating the egg whites on the same medium low speed until desired stage is reached. About 5 minutes for soft peak stage, and 10-12 minutes for stiff peaks.