When you come across a locked terminal you will have the option to hack it to access its features. Do this whenever possible. This guide said it all extremely well, so read up on that on how to successfully hack and use the same guide to find the locations of enough terminals for the achievement.
Note for PC Users: When you find an unlocked terminal, open the command console and click on the terminal. The name and code for the terminal will appear at the top of your screen. With that still onscreen, type "lock" to lock the terminal at the easiest setting. Now go ahead and hack it. Once it is hacked, relock it and repeat until you have the achievement.