Don't step up too early

 This is a situation the Full back will be in quite often. 

I am going to explain the situation as a left back. So the play is shifting from the left right side to the left side, the left CAM has the ball and is about to play the ball to the opponents right winger. Now as a full back, you have the urge to close down the winger as early as possible, so you step up and leave your line early. Now space has opened up behind you and all the midfielder has to do is slot it behind between you and the LCB. The winger is now free and can deliver an unchallenged cross.

You have to stay with your defense line until the CAM releases the pass to the winger. Once he does this you can try to win the ball before it reaches the winger or just hold him off there and force him to play a back pass. Even if you ask the LCB to cover you there will still be a higher chance off the winger receiving it behind you. 

That's it