Attack the second ball at a corner. Position on the edge of the box, read the clearance, and strike the second ball first time and low back toward goal through the crowd.
Take a corner into the box where a defender heads it clear, but only as far as the edge of the box. An attacking midfielder positioned there strikes the second ball first time back toward goal, keeping it low through the crowd. Emphasise the position for the second ball and a clean, low first-time strike. Balance the rest-defence behind. Rotate roles.
Most corners are cleared. That is the normal outcome rather than a failure, and any routine built on the assumption that the first delivery produces a chance is built around the exception. The useful question is what the attacking team has arranged for the ball that comes out, because it comes out most of the time.
A defensive header from a crowded six-yard box travels fairly predictably: up, and out towards the edge of the penalty area, roughly along the line the ball came in on. That predictability is the entire opportunity. A player standing there before the delivery, watching rather than joining the crowd, is in the best position on the pitch and is usually unmarked, because defending sides commit almost everyone to the box.
The strike has to be first time and it has to be low. High is wasted, since there are eight bodies between the ball and the goal and every one of them is a chance for a deflection upwards. Low and hard through a crowd is difficult for a keeper to sight, and a deflection off a leg on a low shot still tends to finish on target.
The trade-off is real and worth telling players. The man on the edge is also the team's insurance against the counter-attack. A side that puts two there is covering the break and attacking the second ball with a spare man. A side that puts nobody there is doing neither.
Ask for accuracy over power. A hit-and-hope from twenty yards through traffic is the most common thing that happens here, and it almost never scores.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.