Mix the water slowly into the cornflour until it forms a thick paste like toothpaste. You can also add food colouring to the water before mixing it into the cornflour for a coloured Goop.
I always place it on top of a wad of newspaper to reduce the mess and to make cleaning up easier.
My kids like to get their plastic dinosaurs and pretend they are stomping around in pre-historic muddy puddles. Great messy fun!