Use a double boiler or create one using a bowl and pan. Fill the pan with ~1” warm water and place a bowl over the pan so it can heat the egg whites with the steam, but it should not touch the bottom
Add egg whites, sugar, and cream of tartar to the bowl. Place the bowl over the warm water. The pan should not touch the bottom but be heated by the steam
On low, heat the egg white mixture stirring constantly until gently warmed and the sugar is dissolved. Be very careful at this stage to not let the egg whites cook. Scrape the sides of the pan, stir constantly, and remove when sugar dissolves
Once dissolved, add in the vanilla and whip the egg whites on high until stiff peaks form. The mixture will be very glossy and thick. When you place the whisk into the eggs and pull up, it should create a peak.