Take off the heat and quickly fold in the cereal mixture. Press into a tin lined with baking paper. (Mine measures 32cm x 20cm.) Cut while still a bit warm, although if you have a really sharp knife you should be okay, because I forgot to cut mine until I got to this bit of the recipe!
You can replace the cereal with popcorn, which is really nice. I have a popcorn popper and it's cheap as chips to pop my own and make this up. You can add coconut (I don't because my kids aren't that keen on it), any dried fruit like raisins or craisins, and some nuts or seeds. Its up to you. Go for it! Beats paying a fortune for made up ones, and at least you know whats in it!