Some hints that I have read. Don’t skimp on the sugar. Use room temperature egg whites. Use 1/8 tsp cream of tartar per egg used.
3 egg whites, room temperature
3/8 tsp cream of tartar
3/4 cup sugar
1/2 tsp vanilla (optional)
Start whipping the egg whites until frothy. Then add in the cream of tarter (1/8 tsp each egg white). Once starting to get creamy looking, start slowly adding the sugar and vanilla. Whip until stiff peaks form.
Carefully create little cookies with a spatula. You could put it in a piping bag, but I usually don’t. Bake at 250°F for 1 hour. Then turn off the oven and let them sit over night.
They should be dry on the outside but marshmallowy on the inside.