Synchronisation of clocks How can two clocks be synchronised with each other at some instant without being at the same place and same time $?$ considering that simultaneity is a relative concept .
 A: Here's the standard way in flat spacetime.  Let's say you want to produce a synchronized pair of clocks that are a spatial distance $d$ away from one another, then perform the following steps:


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*Construct two identical clocks such that they start ticking when they receive a special light signal.  Call the clocks clock $1$ and clock $2$.

*Before you engage either clock with the light pulse, set clock $1$ to time $0$ and clock $2$ to time $d/c$ where $c$ is the speed of light in vacuum.

*Still before you engage either clock, move clock $2$ a spatial distance $d$ away from clock $1$.

*Send out the special light signal from right next two clock $1$ toward clock $2$ so that it immediately starts clock $1$.  
The signal will reach clock $2$ at precisely the time $d/c$ later, so once clock $2$ is engaged, it will be synchronized with clock $1$ for all later times.
A: 
How can two clocks be synchronised [...] considering that simultaneity is a relative concept

The question, if I understand it correctly, is in other words:
Since it seems popular to say and emphasize that "simultaneity is the property of two events [...] what is simultaneous in one [inertial] frame of reference will not necessarily be simultaneous in another." --
is it nevertheless correct to say that "two given clocks had been running sychronously in some particular trial" (or "two other given clocks, in some other trial had not been running sychronously") as such; i.e. without adding the qualification/relativation "in which [inertial] frame" the statement does or doesn't apply?
The short answer is: yes, it is.
(For example: the method described by joshphysics, as presently stated, apparently comes without any explicit disclaimer such as "this only applies in some inertial frame but not in others".)
Somewhat more profoundly, it is important to distinguish whether considerations and terminology refer to individual participants (Einstein wrote in this sense of 
"material points"; MTW, box 13.1 of "principal identifiable points) or to entire events.
Each event is generally understood to involve several/many distinct participants; where each distinct participant its own distinct indication (or "reading" or "position of the little hand, a.k.a. time").
Both descriptional perspectives (either emphasizing events, or else emphasizing individual participants) have their advantages and uses;
and it is accordingly just a matter of descriptional perspective to say that either


*

*"simultaneity is a relative concept", in the sense that two given different events which are space-like related to each other cannot be called overall "simultaneous to each other" or "not simultaneous to each other" but, at best, only "simultaneous to each other wrt. some particular inertial frame, and not simultaneous to each other wrt. all other inertial frames"; or

*"simultaneity is an absolute concept", as a definite relation between one particular indication of one particular participant and one particular indication of another particular participant; at least if these two were at rest to each other in the corresponding trial.   
Now, when considering "synchrony of readings" it goes almost without saying that the "participants perspective" is adopted: it is obvious that the "readings" of some particular participants are meant, and (as far as those participants were indeed at rest to each other) clearly their own common (a.k.a. "proper") inertial frame is of interest. Consequently, "synchrony of readings" is readily understood as an "absolute concept".
Finally, while the "events perspective" seems to be more popular (at present, and arguably already in Einstein's writings), the "participants perspective" appears more suitable for explaining/defining what's meant be "(inertial) frame" in the first place.
