Concrete is a mixture of Portland Cement, sand, gravel and water. As an engineering material it only works in compression. Any tension and the concrete will crack. Concrete structures are designed with metal bars or cloth inside to support the tension loads of the structure.
Concrete can be mixed from the scratch raw materials (above) or can be bought already pre-mixed needing only the water added.
When mixing concrete add just enough water to make the concrete just loose enough to be pushed into the desired form (mold). If anything, on the stiffer side is better then on the runny or loose side.
Just after pouring concrete a screet is used to level the concrete to the top of the form. A screet is usually just a very straight piece of long wood. The screet is placed on both sides of the form and pushed back and forth while being pulled in one direction. This levels the concrete. The excess concrete usually overflows over the side and at the finish of the screeting process.
Then after the concrete has set up partially (enough so that you are able to press one of the grave stones down into the mixture but it requires some force) a concrete float (usually aluminum or magnesium) is used to press the gravel down into the concrete and "float" the portland/sand to the top. This leaves a nice smooth top. This can be done once to serveral times and for a finer finish and a fine spray of water can be added to the top of the setting concrete just before the second or subsiquent floating proceedures. This additional water also help the curing process. It is also at this point you can press your hand into the concrete to preserve you hand print for a very long time, or sign you name in the concrete.
It should be noted that concrete does not DRY. It cures. It will cure to a harder product if youkeep the surface wet for a few days. A wet carpet on the surface works well.