They are fitted together and held in place using elastic bands and ribbons. If made really well they can easily fool people into thinking that they are looking at a genuine cake until they get close enough to realize that the cake itself is actually made out of diapers. Some are made using non-disposable diapers but the majority make use of the disposable kind. Usually white diapers are used by nowadays many of them are available using colored ones. This is useful if you know whether the baby will be a boy or a girl.