In order to be able to throw sand, you must get trained by the Combat Trainer in Monteriggioni. Talk to him, choose Special Moves, and choose Unarmed Combat, it is free to learn.
To do it, you will just need to hold X when unarmed, and release.
You can use the method in this video, get into a fight, use a Smoke Bomb, and let them have it.
You have to have targeted the enemy in order to throw sand at them.
The Sand Throw is at 00:58.
Video is not mine, credit goes to MrBlingBlingNr1 on YouTube.