Cut out a 4' x 4' slab of 1"-thick insulation foam. Consider affixing it to a plywood table structure for added support (you can also glue it to MDF hardboard in a pinch). Use a permanent marker and draw the major guidelines (floor sections, stone tiles, etc.). Consider drawing out your plan on a piece of paper first, so you can figure out all the correct measurements. Also, note that since a Black Templars' keep has very regimented architecture, you might find it easiest to draw a grid pattern with a small marker or pencil to better transfer your designs from the paper. We drew a grid with 1' x 1' sectors.

Mark off where the pillars will go, along with areas of stonework that might differ in texture.

For the more intricate patterns (like the Black Templars cross), draw them out to-scale on a piece of paper first. Then, use a mounting spray (found at most arts & crafts stores) to temporarily attach the paper to the foam. Now, you can etch the design into the foam by making your cuts straight through the paper without it shifting.