Here’s where I experimented a bit. I tried two different mixtures:
Cement + water: This gives a smoother, shinier finish but dries faster and is harder to sand.
Cement + sand (1:1) + water: Darker and more porous, but easier to shape and sand.
I ended up using both for the house in the photos.
The mixture should be thick, like yogurt, so add water slowly until you get the right consistency. I poured the cement into a plastic bag for control.