Fill your plastic tub about half full of dry cement. Pour in a little water at a time and mix it into the cement with a hoe. Add enough warm water to make a paste the consistency of very thick oatmeal or cookie batter.
Spray the inside of a cross mold with cooking spray. Rub the inside of the mold with paper towels to distribute the spray evenly and soak up the excess. The inside of the mold should be shiny and greasy looking, but there shouldn't be any puddles of cooking spray.
Pour your concrete into the mold. Pat down the top of the concrete with a piece of scrap wood. You can also scrape away excess concrete with the flat edge of the wood.
Allow the concrete to harden for about half the recommended time. Use popsicle sticks to scrape a design into the surface of the concrete. This may be Celtic knots, a geometric design, names or even a verse of poetry. Personalize the cross as you see fit.
Allow the cross to harden completely. Turn the mold over onto a padded surface, like grass or old carpeting, and pull up on the edges of the mold. The cross should pop free.