You will receive the drum can during Act 1: Middle East as part of the story, so don't worry about having to search for it. You'll know when you have it as you'll get the usual tutorial pop up on your screen as you do every time you collect a new item. Put it in one of your inventory slots (press Start, go to Items and press X, press right on the D pad, scroll down to Drum Can and press X, then scroll down to a slot that says None and press X). Then resume the game, hold L2 and use left or right on the D pad to select the Drum Can and release L2 to use it. Snake will hide inside it, then walk near to an enemy and press X to start rolling around. Roll into the enemy to knock them over and to get the trophy.
Please vote positively if this guide was helpful and feel free to leave a comment if I can improve it in any way.