Using kitchen shears, carefully remove the backbone from the bird by cutting up one side and then up the other.
Press on the ribcage to open it slightly.
Using a butcher knife or a large kitchen knife, cut the sternum by holding the knife handle and carefully putting even pressure on both the blade end and the handle. Keep the heel of your hand well away from the tip end as it may slip off and slice you.
Once the sternum has been cut slightly, press on the ribcage again to finish breaking it.
Turn the bird over and prepare it for roasting.