First divide and colour your fondant icing. You can use ready-coloured fondant or colour your own with good quality gel food colourings. The six colours I used for my rainbow were purple, blue, green, yellow, orange and red. I used about 100g of each colour, though obviously needed a less for the smaller inner colours and a little more for the outer colours.

Decide which order you want your rainbow colours to be in, then roll the first colour into a long even sausage shape. Don't worry if the ends are a little uneven as these will be trimmed off at the end.

Bend the sausage of fondant around a small circle shaped cookie cutter to get a neat curve for the base of your rainbow. I used the second smallest cutter from my set of round nesting cookie cutters.

Repeat with the next colour of fondant, carefully bending it around the first arch. Don't press the fondant pieces together, as they will be separated again when applying to the cake - just gently bend them around to get the correct shape.

Repeat until you have used up all six colours. Carefully remove the cookie cutter, leave the arch shape intact.

Use a ruler and a sharp knife to trim the ends from base of the rainbow. Be careful to get as straight a line as possible for the perfect rainbow shape.

Carefully transfer the next piece of rainbow into place, resting it on the first arch. Again, the ends of the arch should stand on the cake board and you should press the whole arch gently into the buttercream to help hold it in place.

Repeat with the remaining colours. As this is quite a large rainbow, you'll find that each piece will start and end a little further around the cake, so the whole rainbow will end up wrapped around the front half of the cake.

Once all of the fondant colours are in place, gently press the whole rainbow to make sure that it is even and held securely in place by the buttercream.