Using a jig saw or table saw, create the base for the mold by cutting your melamine board to 27 x 14”.
The inside edges need to be caulked where they meet the base to prevent sand from getting underneath, but your longest side that is closest will be the side getting removed when you demold.
Measure and mark a line ½” up from the base on all inside sides of both forms. It will be easier if you mark a whole line across the sides, this is where you will stop the sand.
Start by combining the sand and cement at a 2:1 ratio. You will only mix enough for one tile at a time.