Use three attackers to put crosses over from different angles with a fourth attacker putting pressure on the keeper. The inside attacker should start passive, then work their way up to actually attempting to head the ball in the goal. The keeper should secure the crosses and then immediately pass the ball out to an outside attacker and be prepared for the next cross. Use multiple balls to keep the action moving constantly. With a larger group you can add in more inside attackers and a lesser number of defenders.