If you are choosing to make the meringues (many bakeries have their own, just do your best to purchase freshly made, so they are terribly hard all the way through, and only on the exterior), follow these instructions.
Heat the oven (convection or fan) to 225 degrees Fahrenheit (or 100-120 celsius). Let the oven preheat while you make the meringue.
Whisk the egg whites either by hand with a whisk or with a hand mixer with the whisk attachment. Whisk until stiff peaks form.
Add the caster/granulated sugar gradually to the egg whites, mixing in a little at a time until stiff peaks form each time. This may take some time. Be sure to use the freshest eggs you can find. While they will take longer to whip the proteins of the egg whites into a foam, when the foam finally does form, it is firmer and more stable. In other words, keep whisking (this is where a hand mixer or mixing stand becomes the favorite kitchen tool in your kitchen ☺️).
Line a baking sheet with parchment and using a large spoon, place large dollops of the meringue on the parchment. Create peaks so you have texture (but even if your meringue is not as stiff as you would like, cook them anyway. they will still taste amazing).
Bake on the lowest rack in your oven (make sure it is on fan or convection oven) for 1 hour to 1 1/2 hours. The test to determine if they are done - they are sold - entirely firm and easily come off the parchment . Let them cool for at least 30 minutes, but even longer would be helpful as you are going for the crunch that the cooling creates.