I’m making a hey duggee cake a bit like this one but with the big character and 5 small ones around the cake.

An edible image would probably have been easiest but I’ve left it too late. So how to do all the details? Cutouts in white, then each part cut out in colour and layered on? They would be flat rather than curved round the cake. Or cutouts in white and colour in with edible marker? Or cut out parts in colour and stick directly on the cake? Or some other method?

I would just do them on color and stick them directly on the cake with a little water.  If you stick them on before they dry they will bend with the curve of the cake.