They are so small in fact that they float around in the atmosphere, eventually reaching the ground with rain droplets or heavier dust particles. The best times to collect micrometeorites are after meteor showers. The dish needs to be very clean. Let it sit outdoors long enough to fill. If you don't live where it rains very much, fill the pan with distilled water and let it sit outside for a few days.
Metallic micrometeorites are usually iron or iron and nickel and can be collected with a magnet.
Use a small but strong magnet and cover it with a small plastic or cellophane bag. The idea is to provide a barrier around the magnet that can be immersed in water. Sweep the covered magnet slowly through the water along the sides and bottoms of the dish. Remove the covered magnet, and place it into a second clean dish filled with distilled water. Once evaporated, magnetize a straight pin or sewing needle by rubbing it one way across a magnet for a minute or so.