Spray your mold with cooking oil and set aside. Mix the concrete so it is about the consistency of peanut butter, not a slushee. It should hold a bit of form when shaped into a ball, but not so wet that it slumps, and not so dry that you can't make a ball without a great deal of effort. Not too wet, not too dry.
Pack the mold, tapping and poking the concrete down into any crevices in the mold. Once filled, level off the top with something flat like a ruler. If you can, carefully bounce the project on a firm surface that is covered with something cushy. You don't want to break the mold, especially if you've chosen a glass mold.
Set on a level surface out of direct sun. Wait twenty four hours before you even think of touching it.