When you choose a head on minecraft-heads.com, it will show you the commands for a number of ways to access the head in-game. We need almost none of this but we need a couple of values out of them. I've just been pulling the first give-code field it shows you, which will give you something like this:
 
 /give @p minecraft:player_head{display:{Name:"{\"text\":\"Rose\"}"},SkullOwner:{Id:"50af25ef-adb9-44d0-880f-6890ea0dc182",Properties:{textures:[{Value:"eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvNTVkODk0MzFkMTRiZmVmMjA2MDQ2MWI0YTM1NjU2MTRkYzUxMTE1YzAwMWZhZTI1MDhlODY4NGJjMGFlNmE4MCJ9fX0="}]}}} 1
 
 
 We need two of these values. The ID (SkullOwner:{Id:"<all of this, no quotes or carrots>") and the value ({Value:"<all of this, no quotes or carrots>")
 
 Once you have the skin you want, add it to headSkins.dsc like so
 
 <name>: <ID>|<Value>
 barley: 2336f19d-2f60-4e5b-b969-be7ff0456f7e|eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvMThlMjJmZWM1ZmI2Mzg5ODlhNjY3Y2QxNzM3NzFjYzliNTJlNTI5Yzg2N2JkNGExMWI4NzNjNDI3NzVhZDU2ZCJ9fX0