How Itachi was able to use Shadow clone?
It was never confimed in the cannon of Naruto-verse. However it can be inferred that such a technique can be self taught.

Naruto did it.  

With a shinobi as good as Itachi, who mastered the art of Sharingan at the age of 8, and having joining Anbu at the age of 11 Itachi wiki, it shouldn't be hard to self-teach the B-ranked Shadow-clone jutsu. 

Shadow clone Jutsu not imposing any blood-line limits (kekkei genkei)- and  Itachi being a prodigy above all others-  he could have copied the jutsu with his activated sharingan.