Set up with your two center backs (yellow team) in a box 30 x 20. Give them three or four outlets (two outside backs and one or two center midfielders). There is also a keeper in a goal 18 yards behind them. The outside backs are positioned wide and the midfielders are in the middle (both groups outside the grid).
It is preferable that you set up your grid at the top of the 18 for realism.
I have two defending forwards (blue) that apply pressure to create a realistic environment. Their starting position is at the top of the grid as well. If they win the ball, they are trying to score.
The game starts with service from beyond the top of the grid. The center backs have to decide if they have time to settle and play, knock it down to the full backs, find the midfielders, use the keeper or clear.
Clearing is the last resort for me. The objective is to play until the midfielders have received the ball and found the servers feet. The server also functions as a target forward.
You can progress to a more realistic playing area, with more defending attackers in the form of midfielders. You can also have the target marked by a player. (Use another server at this point in the form of a second keeper.)
If you have the players available, you should be using two keepers. The center backs now go from dealing with pressure to a build up that leads to a scoring opportunity.
You can progress from this soccer drills to an 8v8 game to see if there is any transference from the initial activities.