I came across this idea at the home-de-pot when I found in the cement section plastic forms for making walkways. They did not have a form that I liked or one that was big enough so I decided to make my own.
Step 1: The form
I built a simple mold with 1/4" X 3" poplar stock from the local big box hardware store. Then I used 1/4"x 1/4" square stock to make a decorative pattern. The whole thing was held together with grabber screws. I used Vaseline as a release.
Step 2: The cement
It took approximately three(80 lbs) bags of cement to fill one form (3 foot x 3 foot x 3 in) so after I got the form in place and level I mixed the cement coloring with the water and then mixed the cement into a fairly workable texture. Then pour the cement into the form and work it around until all of the air pockets are worked out. It is important not to leave any aggregate from the cement on the top of the form, this will make it harder to remove later.
Step 3: Form Removal
Wait around two hours for the wet cement to harden enough to allow the form to be removed. Gently tap on the surface of the wood form to release it and slowly work the form up and off. There was some cracking but dont worry about it. The mortar will fill in any voids.
Step 4: Mortar
Finally after allowing all the cement to cure I swept in the mortar into the cracks and wet it down gently with the hose. The mortar ties the whole piece together and reinforces the cement as well as filling any voids.