This is a Simple Trophy to Earn :D and is done in the Specialists HQ (Solo)
it Requires you to win 1 Skirmish with Each of the specialists on each of the 3 difficulties
1) Recruit (Bronze Star)
2) Regular (Silver Star)
3) Veteran (Gold Star)
In Total it will mean you have to play a total of 30 Skirmishes, which can get boring after a few
so take it slow and do 1 or 2 here and there in between playing multiplayer/black-out or zombie matches
There is no "Change Difficulty" button/option
The difficulty changes automatically when you play the skirmish
The First time you play the skirmish, it will be on recruit Difficulty, Second time, Regular, Third Time, Veteran :D
Hopefully this helps :D i know the description of the trophy doesn't explain it too much :D