Preheat the oven to 175 degrees (without the fan if possible)
Place a layer of silver foil over a cold baking tray
Iron your crisp packet (I put it under a towel)
Place the crisp packets on the tray and then place in the oven
Watch as they miraculously start to shrink (you think nothing is happening and then POW all of a sudden they start)
As soon as they stop getting any smaller remove from the oven and place a heavy book over them to ensure they stay flat
I punched a hole in the packet before placing in the oven for the one we turned in to a keyring. We then put an old keychain through the hole.
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