Keep in mind that the cement will pick up the surface texture of the mold.
That's another reason sand is popular.
It can be packed down to a fairly smooth surface, and what sticks to the cement can be wire brushed off, or wait a few days and pressure wash the stones.
If you want a negative image in the cement (carved into it), all you need is a casting of that object made of a water soluble material. No undercuts is always a good idea, but in this case not too much of a concern as you dissolve the positive away.
Say you want a skull face carved into the stone. Make a mold of the face from a flexible material backed with a rigid mother mold.
Now make some jello. Yep, jello. The color might stain the cement so unflavored (and uncolored) gelatin is probably best.
Use the "Jello Jiggler" recipe from a box of flavored jello, just add one extra ingredient, sand.
Basically you use the gelatin as a binder to hold the sand together. The mixture should be fairly firm, nearly all sand.
Fill your mold with this, being careful to avoid any air pockets.
After the gelatin has set, pop out your skull face.
Trim the back flat, at the angle and depth you want the carving to be. The deeper the carving, the more likely that the gelatin will become distorted from the weight of the cement, so don't go too deep.
Place the gel/sand skull face in your sand mold, toothpicks can help keep it in place if you are worried it might shift.
As with the clay letters, when you demold, a spray with a garden hose should remove any trace of the gel/sand mix.