Making a Concrete Floor for a Diorama

Make a frame from strips of wood to the required size and glue them (using wood glue) at the corners.

In this case, the height was 7mm. 

After having put the frame on top of the planned diorama base, in this case a piece of wood, measure the inside dimensions of the frame, and glue a matching piece of 60 grit sandpaper inside the frame. Ensure that it is flat on the base.

Mix some building cement, cretestone and water to the consistency of porridge.

Rather make more than you think you will need - it's cheap enough! (I used 1/2 cup of each and added water as required).

Pour the cement mixture over the sandpaper and roughly level it out. 

Then gently and rapidly bump the base on a hard surface to release any air bubbles.

Spray some water over the top using a mist sprayer. Then take a straight plank/ruler/etc and move it across the surface, at an angle and with a tapping motion from left to right (or righ to left if you want to!) Do this until the surface is super smooth and even.

Leave to dry completely. In the photo above, it looks a bit bumpy, but it's actually not. I think it was the lighting. 

Cut between the plank frame and the cement before it is completely dry and then remove the plank frame.

To add cracks to the "concrete floor", simply twist the whole base gently when the cement has half-cured. The cement slab can also be sanded once dry, if needed.