Read your opponent

 I'm still new to this part of the game. I think you need to give yourself time to watch the player. But it can't be too long as well, from what I know 4-5 minutes into the game and you should get the basic Idea of how he/she plays. However it could take more time if they have a huge arsenal of skills and stuff they do.

Things I think I should be looking for

1. How fast they move: to see how much distance to guard them from.

2. How tall are they: also to see how much distance to keep.

3. Which foot are they using more (the preferred foot)

4. Habits: what do they like doing? If it's a winger do they want to cut in most of the time or go out. If it's a fullback does he want to force me back or is he trying to show me a certain side. If he is showing me a certain side, maybe I should fake and go to the side he's covering etc. 

5. Formation: see what formation they're lined up. (this has to happen during kickoff so that you get the best and easiest time to watch everyone. Knowing the formation might give some information to you based off of your knowledge of that formation, you could use it for counter pressing. 

6. Tactics: are they trying to hit long balls? do they want to play directly to the midfielders? are they building from the back? do they want to play the ball out and back to the center? etc. Knowing the game plan through patterns might help to understand the player your markings role in their game.

That's it for now I 'll keep adding to the list.