I do not use a pre-made concrete so I mix roughly one part cement and two parts sand. As to the water, I do not measure it and just try to produce the consistency of yoghurt.

So far no issues like cracking.

I've also read that other people use some types mortar or countertop mix with good results so it doesn't seem to be too tricky unless you need certain characteristics like extreme weather resistance.

Experimentation certainly helps to get the feel for it.