Make it as close as possible to the moment of serving, it's always nicer freshly made.
Make sure that the pot, bowl and whisk are very clean and without any traces of fat. This can avoid the meringue from firming up. If you're doubting yourself, give everything a wash and dry thoroughly.
Place the egg whites in the bowl where you'll be making the meringue. The sugar and water go into the pot.
Bring the sugar and water to a boil over medium-high heat and let it reach 116-118°C/240-244°F (you'll need a kitchen thermometer). From the moment it goes over 101°C/213°F it increases steadily so keep an eye on it.
When the syrup reaches 110°C/230°F, start beating the egg whites at high speed. Once it reaches the temperature we need, remove the pot from the heat and add it slowly down the side of the bowl while still beating at high speed. We make it go down the side of the bowl so that it doesn't impact the whisk of the mixer.
Keep beating until it has completely cooled down to room temperature.
Spread it out on top of the pie with a spatula or use a piping bag fitted with a tip of your choice.
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 for a couple of minutes. You can also just leave it white.