Anything between nothing and "Hold the nail in a fixed position against the wall without moving it so I may strike it with my hammer"
How much detail do you think you need to give. It is quite possible that you don't need to say anything at all. The mere fact you are working together means that there is no need to say anything at all. Or "Hold it, please" might be enough The answer is language independent. If it is not clear, from the context, that it needs to be still you can add that word "Hold it still, please".
If you need to tell the person that it needs to be in a particular place, (because that is not clear from context" You can add something like "Hold it here, please" or "Hold it on the cross that I drew on the wood".
How much trust do you have in the other person? Do they understand what you are doing? Do you need to explain to them. "Hold it with the point down"
However, all your proposed sentences are good and correct. They tell the person it must not move, and imply that you expect that the person will move the nail if they are not told to keep it still and fixed.