Casting with cement can be fun and easy if you into the process with the right attitude. I started using epoxy resin about a year ago with silicone molds so using cement is a lot cheaper. For what I paid for resin I could get more than quadruple the amount of material out of cement. So if you're cheap like me, but like to experiment cement is a really good option.
Step 1: Step 1: the Mold
You can use many different kinds of silicone molds. A great place to scope out molds in the bakery or confectionery section of your local craft store. A lot of my favorite molds are silicone ice cube trays.
For this instructable i'll use on of my own original mold that I made with SmoothOn's oomoo 30.
Step 2: Mixing Your Cement
I like to use plain Portland cement or mortar for my final piece. I find that using concrete can be a challenge for smaller pieces because the aggregate in the concrete can settle in the mix and ruin some of your detail.
For Portland Cement I use a 1 to 1 ratio cement to water. When you mix them together they should be the consistency of oatmeal.
Step 3: Pouring the Cement
When you pour the cement you should do it slowly, bit by bit into your mold. It's a little chunky and you'll clog up the opening to your mold if you do it too fast.
While you are pouring you should be vibrating your mold to allow the bubbles to escape and for the cement to fill in the open spaces. You don't need a specific tool for this, some people will run a sander on the table. I like to tap my mold on the side. If you have any overhangs you may want to pick up the mold and wiggle it around to make sure your mix is everywhere.
Step 4: A Lot of Detail
You can get a surprising amount of detail in the cement. It really takes patience and experimentation. You might want to experiment with sand and aggregate to get a certain texture you're looking for.