Basically as we say in the trade. (You got to kill it). Meaning= over do it with the cement. Normally for a cement mix,you would mix half BUILDING SAND, half LIME STONE with two thirds of a bag of cement. But if you want it rock hard, then i would put the whole bad in. Make sure it is of the consistancy of porrage not with the emphasis on it being not to runny.
Then after pooring it, you have to tamper it to get all the air to the surface. You do this by laying a plank of wood over the top and forceing it in the concrete whilst lifting it back up again for 5 to 10 mins.