First decide what you want to copy. It must be something that can be painted with Acetone without melting. An example would be a brick or rock.
Now get some of that blue or pink rigid foam insulation. You should try to get the 2 inch thick stuff if you want your mold to last. To begin just get whatever you can. Sometimes you can pick up a couple of scraps at construction sites for free.
Clean your rock and be sure it is dry. This is important; water and acetone do not mix!
Put your insulation on the table, and next to it put your rock with the side you want to copy facing up. Paint the rock with Acetone and VERY quickly place it on the foam. When it has sunk in an inch or two pull it out.
Wait a while for the foam to harden up, and there is your mold!!
Paint on some mold release (also known as 10w engine oil). Try not to have any puddles; they will cause holes in the concrete.
You may have to play with this a bit to get the feel of painting and sinking your rock, but it works!!