You'll need a fairly sensitive scale for this procedure. A bathroom scale won't cut it!
Crush ½ cup of cereal in a baggie, until the flakes are half their original size. Pour into a bowl.
Add 1 cup of hot water and mix with a wooden spoon.
Get a strong, 3-inch bar magnet that is not grey or black (so the iron filings will show up). Don't use a horseshoe magnet.
Put the magnet into the cereal mix and stir gently in a circle for a fixed amount of time, say 5 minutes. Try not to bump the bottom or sides of the bowl.
Take out the magnet. Remove the iron filings that it pulled from the cereal, and weigh them on a laboratory scale.