The earliest you can get this is in Act 2: South America as this is the first time you can buy them in Drebin's Shop. You can access his shop through the pause menu and the Emotion Mag is in the Etc. section (press right on the D pad six times). Its costs 2000 Drebin Points. Drebin Points are the currency in this game and are obtained every time you pick up a gun you already own or some ammo that you are already full of.
You will need to buy or find one of these magazines and place it in an enemies patrol route. To place it you need to first equip it in a weapon slot from the pause menu (make sure you choose weapons, as it's not classed as an item!) Then equip it and press L1 and R1, Snake should crouch down and place the magazine on the floor. When an enemy gets near it they should get a ! above their head and start reacting to whichever beauty is in the magazine (e.g. if the page shows Laughing Octopus, they will begin to laugh). After about 30 seconds or so the trophy should unlock.
I found the easiest way to do this was to find a lobe enemy, sneak up behind him and place the magazine, then hide around a corner and run in a small circle so he could hear my footsteps. As soon as he started to walk in my direction I help X to go prone and the enemy walked to the magazine and started to read it. I did this right after the fight with Laughing Octopus.
Please vote positively if this guide was helpful and feel free to leave a comment if I can improve it in any way.