You need a mold. Commercial molds are available at craft stores in circles and squares and other shapes. Or you can make a wood-framed mold using 2x4s and nails to create a square, hexagonal or triangular shape — really any kind of straight-edged shape you desire. An old circular tray or large circular baking pan can create good-sized circle stones, as can an old bucket or pail (cut the sides down so you have easy access on both sides).
Mix packaged concrete in a wheelbarrow with water, following package instructions. Remember to stir well with a shovel, adding small amounts of water at a time. Mix it until the concrete's texture and density is homogenous — not powdery and dry, nor thin and watery.
You can add color with QUIKRETE® Liquid Cement Color. Add an ounce or a bit more (depending on how much color you want) into your concrete mix and stir it around until the color is consistent throughout.
Use some cooking spray to lubricate the inside of your mold so that when the concrete has set and dried, your stone can easily come out of the mold.
Be sure to lay your mold out on a flat surface with plastic sheeting underneath. Pour the concrete mix into your mold. Shake the mold a bit to remove air bubbles. Using a stick or similar object, write your message into the concrete. You can also use concrete stamps after you've let the concrete set for about a half hour. Let it set — or harden completely — for about two days depending on the climate where you live.
Wear a dust mask, eye protection and gloves when working with concrete.
After the concrete has hardened, gently flip the mold over and lightly push or tap the stone free.