We used two boxes of cornflour in an underbed storage box - outside on a table and then on the ground because it seemed easier, especially for my one year old. We mixed in water gradually until it was the right gloopy consistency and then added some coloured ice. I made the ice in a silicon mini muffin tray so they were a bit bigger than standard ice cubes but the normal ones would be fine. I used the brilliant Wilton food colouring gels - unsurpassed in the quality of colour they create!
Clearing up tips!
Once you are finished playing, let the gloop settle for about half an hour, and then pour off the water from the top, without pouring away the cornflour, which sinks to the bottom. Then let it dry out for a day or two. It turns solid, smooth and cracked (see photo below); you can just lift it away from the box and pop in the bin. You could almost turn this bit into another activity, it's pretty fun considering it's actually clearing up! Also - with any spills around where you have been playing, wait till it dries and then just sweep/brush it up. Try not to wipe it at all if you can, that just spreads a thin layer of cornflour over everything.