Once your cakes have cooled place the first one onto a cake turntable or plate and either pipe (I used a large round piping nozzle) or smooth the buttercream onto the top. Don’t worry if it splodges out of the sides we will fix this later.
Place the next cake on top and repeat. Repeat this process until all three cakes are stacked.
Using a cake smoother go around the sides of the cake and smooth out any icing that has splodged out.
If there are any gaps, fill these in and smooth again. Don't worry if you can still see the cake through the buttercream this is just the crumb coat. Pop the cake into the fridge for 30 minutes until the buttercream has hardened.
Once your crumb coat has hardened Pipe the remaining icing around the sides and top of the cake. Smooth with your cake smoother. Any buttercream that comes off on your smooth pop back into your mixing bowl. We will use this to decorate the top.
If there are any gaps fill these with buttercream and smooth again until the cake is completely smoothed.
Add the sprinkles to the bottom of the cake. I did this by putting them in my hand and pressing them onto the bottom of the cake.
Once the sprinkles have been added put the cake back into the fridge for 30 minutes until the buttercream has hardened.
Once your buttercream has set, pipe the remaining buttercream onto the top of the cake. I used a Wilton 1M Piping Tip for this and finish with some more sprinkles.