To melee, make sure you're within lunging distance or right up close and press down on (or if you've changed the controls ) to preform a stab.
Unless you're on a lower difficulty, the best mission to get this on is "Stronghold". Play through until you get down to the prison. At this point Price will switch the lights off, leaving half a dozen guards in total darkness. From here, you can simply walk right up to them and get a free kill. Seeing as you have your night vision and they don't, none of them will see you unless you physically touch them.
If getting this in the campaign is proving difficult, load up the first map in the Spec Ops Survival mode and you can gain 5 quick kills on the first wave.