1. When you look at the bottom of your magnet, you should see pebbles, sand, and slivers of metal sticking to the bottom side. Now put your magnet over the large container and peel the bag away from the magnet, starting from the top (Just to clarify things....keep the magnet inside the bag and use your other hand to seperate the side of the bag with metal on it away from the magnet so that the metal will fall into your container). Now repeat until you think you have enough.
2. Now get your mortar and pestle out. Pour a small amount of your mixture into it. Grind it until you get a fine powder. This will also allow the pebbles with the ferrous metals inside to release them.