Before you spend money on concrete, it’s worth trying to Freecycle some–people rarely purchase just the right amount of materials for a particular project, and even a quarter of a bag of concrete is going to do you just fine.
To mix the concrete, get out an old bucket, an old cup, and a paint stirrer. Measure out an amount of concrete that will fill your mold (Do you know how to fill a mold with water and then pour it into a measuring cup to find the volume? Science in action!), and just enough tap water, a little at a time, to give the concrete a brownie batter-like consistency. This is actually the only part of the project that’s messy, since concrete mix is dusty, so do this on the porch.