How do you get the knife because I do not know how and I want to do some..

You sneek up to the person you want to kill and hit the attack button/knife button and it should work that way good luck

When you see the enemy in prone position (laying down) or posted up waiting for action this is possible. You have to creep (move very slowly) on them. In real life if you run up on someone they can hear your foot steps. Unless you creep. Have patience. The option will appear at point blank range. Press X and enjoy the rewarding kill.

the knif is always with u in all missions but to use it u must hit the enemy from the back while he is not alerted by pressing (x) bottom ... if u layed on th ground u will be unseen so u can stlength easily
good luck

Id say to do this in missions like whipsaw or in highwire or any othe night mission first of all you need to camoflauge and whatch their steps when ]he turns around look for beshes or cover and move very slowly or creep and try to knife him but remember to use stealth and go slow sometimes if you rifle butt them from behind and prees x again and he will knife him

In any level after I finished my 3 ranks then, I used smoke and got behind the enemy, and a knife appears then by tapping your x button, then your man grabs the guy with on hand, then takes his right hand slices his neck, then stabs him in the chest.....