Velvet Assassin

This will probably come naturally through playing through the campaign. Just try and kill every guard possible by sneaking up behind them, or waiting for them to come around the corner. You will know when you are doing a silent kill when the red screen pops up as you are about to kill them, and then you need to push the A button. The kills do stack across the campaign. Here is an example of a stealth kill. Video credit goes to youtube user endmover