Get a pad of the finest-grade steel wool. It will be marked 0000 or #0000. Get the type without detergent.
Put on protective gloves and break the steel-wool pad in half.
Rub the two halves together over a sheet of paper. The strand of steel wool will break apart, causing filings to fall on the paper.
Lift the paper and pour the filings into a dry container.
Hold a steel file in one hand. Put it on top of a piece of paper.
Pick up an iron nail with the other hand, and rub the nail back and forth over the steel file. The filings from the nail will fall onto the paper. The size of the filings will depend on the coarseness of the steel file.
Pick up the paper and pour the filings into a dry container.