Hi, so I'm going to teach you how to get customised heads of any in-game player in Minecraft 1.7+. First off, you need to either,

A) Use an existing world which has cheats on.

-OR-

B) Create a new world with cheats enabled.

Now that you have the world open, you need to open the bar that lets you input commands. (If you don't know what I'm talking about, press "T" by default.)

With the bar up, type "/give "Your Minecraft Username" 397 1 3 {SkullOwner:"Any Minecraft Username"}

When you have that typed out, press enter. You should be given a player head called "Any Minecraft Username"'s Head.

I will put a picture of what the command should look like. Where is says "Pblue7" you should put your username, and where it says "Notch" you should put the Minecraft username of whatever player's head you want to get.

I hope this guide helped!