Clean your tin out thoroughly. Running it under the kitchen faucet and drying with a hand towel will work fine.
Massage the modeling clay in your hands until it is pliable. Because modeling clay is very stiff, this may take quite a bit of time.
Fill your tin with the modeling clay. Use your fingers to work out any bubbles. Make sure that the clay is even.
Set the tin on a flat surface and press your key into the clay. Make sure to apply even pressure. Using your thumbs will help to keep the pressure even.
Remove the key. The modeling clay will now hold and imprint of the key.