Line a plastic container with cotton wool, which can be found in craft stores or online. Add enough cotton wool to line the bottom 2 inches of the container.
Spray the cotton wool with water using a plastic spray bottle. The cotton should be damp but not soaked.
Pour some mustard seeds across the top of the damp cotton wool and set a piece of thin cloth over the top.
Set the bowl on a windowpane that gets partial sunlight. If the bowl is subject to direct sunlight for longer than an hour or so, move it away to avoid killing the seeds.
Periodically check to make sure the cotton wool is still damp a few times a day.
Wait for three to five days, or until the mustard sprouts are at least an inch long. Cut them away from the cotton with a pair of scissors and eat them as you please.