First, mark a 10×9 ½” square within the 11×10” base piece. Then trace the locations of each piece onto that.
Since you are casting this in reverse, the concrete cast against the base part of the foam board will be the top of your brick. The bottom will be the concrete that will sit at the top of the form (exposed to air).
I used about 5 or slightly more cups of dry mix for 2 bricks. For adding the colorant, it will be much easier to get a uniform mixture if you add the red colorant to the dry mix– before adding the water.
To take apart the mold, remove the nails from one long edge, where the dividers are, and from the bottom of the edge.