I already mixed some concrete. I mixed 1 part cement and 3-4 parts sand. Then I added water until I received a consistancy, which is good to work with. I´m using a styrofoam ball, which can be opened and a plastic ball with a hole in it. Now, we are going to cut a hole in the styrofoam ball, in order to fill the concrete into it. Before we fill the concrete into it, we are going to brush the molds with oil. This will help the dry concrete come out of the mold much easier. Next, we are going to close the molds. And then we are going to apply adhesive tape to the molds, to make sure they stay closed. Now, we are going to fill the concrete into the molds, using a funnel or an icing bag. When the concrete is fillled up, we are going to poke in it with a little stick, so that air bubbles can escape upwards. Then we are going to let it dry for a couple of days or an entire week. I let the concrete dry for about one week and now I´m going to take it out of the molds. Then I´m going to sand the edges with a sandpaper. Now, they look like this. If you want, you can leave it like this or you can paint on it. I drew a world map onto the sphere and now I´m going to paint it with a golden Maya paint. Now, it looks like this. There is now significant difference between the plastic and the styrofoam sphere. And you can reuse both.