If you are using the Dr. Oetker mix, just follow the instructions on the label.
If you chose to make the meringue from scratch, follow these steps:
Add the egg whites, powdered sugar and lemon juice in a heat-proof bowl, on bain-marie, on low heat. Make sure the bottom of the bowl doesn't touch the water surface.
Gently whisk the mixture for 2-3 minutes, or until it is hot to the touch. This will "cook" the eggs and make them safe to eat.
Remove from heat and process with a mixer until firm. Transfer on top of the tart and use a spatula or a spoon to evenly distribute the meringue and form stiff peaks.
Use a cooking torch to slightly toast the surface by making small circles and moving it over the meringue.
Cut using a knife with a hot blade (dip it in hot water and wipe afterwards) for a clean cut.