Add the icing sugar and almonds to a food processor and whizz for 2 minutes.
Sieve the mixture into a large bowl.
Put the almond mixture back into the food processor and whizz for a further minute.
Re-sieve the mixture into the large bowl and leave it to one side.
Whisk the egg whites until they form soft peaks. I did this using the whisk attachment on my KitchenAid, but you could use a hand mixer too.
Gradually add in the caster sugar 1 tablespoon at a time. Leaving a minute or so in between each addition.
Once all the caster sugar has been added, continue to whisk the meringue until it forms stiff peaks and is glossy and shiny.
Gently fold the egg whites into your almond mixture until it is fully incorporated.
Press the mixture into the sides of the bowl. This is called macaronage, and you are trying to get the mixture to be as smooth as possible. You know it is ready when it flows like molten lava from your spatula.
Add the mixture to your piping bag with a small round nozzle and pop to one side.
Line your baking trays with a macaron mat or baking parchment.
Pipe the batter onto the mats or baking parchment. You need to hold the piping bag completely vertically, or you will end up with wonky feet. Pipe the macarons around an inch or so apart so that they don't touch
Tap the trays on your worktop to knock out any air bubbles. If you see any air bubbles on top, gently pop these with a skewer.
Leave the macarons for 45 minutes to 1 hour so that a skin forms and they are completely dry to the touch.