The first thing you need is the achievement unlocker.
Virus Scan: https://www.virustotal.com/ar/file/7ae4930760e809dc67c3e518f4af7caea525f915513b2509561ffe84ebd49c4c/analysis/
Next, Start up Steam and open up either SAM.Game.exe Or SAM.Picker.exe.
They are both the same so it doesn't matter which one.
For This tut I will use MW2.
Once you open it up you will see all the games in your library.
Then select your game.
After that hit the unlocked lock.
Then all your Achievements will be unlocked on that game!