Make it as close as possible to the moment of serving, it's always nicer freshly made.
Put the egg whites and sugar in a bowl where you're going to mix them afterwards. Put it on a bain-marie. Don't let the bottom of the bowl touch the surface of the water. Use a whisk to mix every couple of minutes.
Once you don't feel any sugar crystals at the bottom of the bowl (touch the bottom of the bowl with your finger) you can take it off the bain-marie. If you have a thermometer, check that it reaches 79°C/174°F. Immediately beat with a static or hand-held mixer until it cools down to room temperature and is firm.
Lay on top of the pie and spread out.
You can make the meringue look a bit golden using a blowtorch or place it in an oven at 200°C/400°F on broil or just leave it white.