Blend one part sifted Portland cement with three parts fine sand in the plastic bucket.
Slowly add water, mixing and stirring as you pour it in. If you are using concrete colorants, add them now. Keep mixing and stirring until you have a mud-like consistency.
Set up the mold material in the shape you desire.
Fill the mold in increments, patting the concrete down as you pour it in to remove air bubbles until the mold is full.
Pull a trowel or a straight length of board over the wet surface of the concrete to smooth it.
Allow three to six hours for the stone to cure. Do not cover. Remove the mold forms when the stone hardens.
Carve designs, dates or personal messages into the stone using metal implements such as a screwdriver or a spoon.
Smooth the outside edges of the stone with a damp sponge.
Allow the stone to sit uncovered for 24 hours. Lightly mist it with water three times during this period.
Turn the stone over and rest it on a soft surface such as an old towel. Smooth the bottom of the stone and bottom edges with a damp sponge. If you're inclined to sign and date the bottom of the stone, do it now with a sharp instrument.
Wet the stone down with a hose. Place the stone in a plastic bag, close up the bag and allow the stone to cure for a week.
Remove the stone from the plastic bag and place it in the desired location.