My personal recommendation is to just go into a playlist, make a very VERY conscious effort to not tense up, especially your hands because that will send your aim all over the place. Don't aim down the sights until your reticle is as close to the enemy as it can be, and only take shots that are headshots. You WILL get slaughtered. Over and over and over some more, but the point isn't getting kills or maximizing your k/d. You are only in those matches to learn how to line up shots and staying relaxed. If you have a friend who will sit in a private match with you, then do it that way. Have them start by just moving around with very basic movements while you try to kill them, then get them to add in jumping and sliding, then finally have them play you for real with gunfire after you've learn to kill them in all the evasive techniques.