Here is how I made my concrete coffee table,
First make a mold out of milamin wood. The size is up to you.
This one is 500mmx1000mmx45mm.
Put silicone inside along the edges. Before pouring the concrete make sure that the mold is very clean and level. Then spray cooking spray inside the mold, or you can use olive oil, just to prevent the concrete from sticking to the mold.