Ideally, separate the egg whites from the yolks 3 to 4 days before, and keep them in the refrigerator. On the final day, leave them at room temperature for at least 1 hour before preparing the meringues.
Cover a baking sheet with parchment paper (or silicone mat)
Preheat the oven to 100°C (210°F).
Pour the egg whites into the mixer bowl with a few drops of lemon juice. Whip at medium speed. When the whites become frothy and have increased by about 1/3 in volume, gradually add the sugar. Continue to whip on high speed until the whites are consistent and form stiff peaks (about 10 min).
Add the sifted powdered sugar, very gently.
Prepare the piping bag (to colour your meringues, see the directions in the text above), and fill it with the mixture. Shape the meringues on the baking sheet.
Bake them for 1 to 1.5 hours. You can open the oven for 1 minute after 1 hour of baking to let the moisture get away. Once the meringues are cooked, turn off the oven but leave the meringues inside for 1 hour or more to let them properly dry.