Preheat the oven to 200ºC / 180ºC Fan / 390ºF / Gas 6
Line a baking tray with parchment and pour the caster sugar into the tray in an even layer. Put the tray in the oven for 5 minutes.
Meanwhile, whisk the egg whites on a low speed until you see bubbles, then increase the speed to medium until the egg whites form stiff peaks. You can check the egg whites are ready by turning the bowl upside down - if they don't slide out, that's stiff peaks!
By now the sugar will be ready. Take the tray out of the oven and reduce the oven temperature to 100ºC / 80ºC Fan / 190ºF / Gas 3.
Add the hot sugar to the egg whites mixture a tablespoonful at a time beating on full speed. Take care not to add any burnt or crystallised sugar from the edges of the tray.
When all the sugar has been added continue to whisk for a further 5-7 minutes.
Test the meringue by rubbing it in-between your fingers, keep whisking until you can't feel any grainy sugar.
When the mixture is ready it will be bright white, thick and glossy.
Divide the meringue into bowls depending on how many colours you'll be using. Colour each bowl of meringue by adding a small drop of food gel and mixing well until you reach the desired colour.
To pipe vertical trees, fit a piping bag with an open star nozzle - I used the Wilton 1M.
Hold the piping bag vertically over a tray lined with baking paper and squeeze the piping bag releasing the pressure one or two times to get a tree shape (bigger at the bottom and smaller at the top).
To pipe flat trees fit a piping bag with a french tip nozzle - I used the Wilton 6B.
Start at the top of the tree and pipe a line underneath, swirling as you go so that the lines of meringue get progressively wider and you have a tree shape.
Cut a couple of paper straws into 4 sections and push a straw into the bottom of each tree.
Add sprinkles and bake for 2 hours or until the meringues are completely dry to the touch and lift away from the baking paper easily.