the notion of relativity of simultaneity

One of the most fundamental problems in the history of humanity and the very basis of the notion of relativity of simultaneity: how to synchronize two watches so that they give the same time at the same present moment?
A little explanation would have been welcome.
Of course, in a Newtonian universe, it is not difficult.
But in a relativistic universe where each observer has his own hyperplane of present time?
A slightly blurred line in Poincaré.
Five lines in Einstein who thereby succeeds in the feat of evading the anisochronous question.
Nothing in Jean-Pierre Messager (the great critic of modern science).
One would think one was dreaming.
R.H.