Time the run to attack a cross and finish with a firm downward header. Meet the ball at pace, get above it, and head it down and across the keeper into the far side.
A wide player crosses from the flank, a striker attacks the near or far post, keeper in goal. Time the run so you meet the ball at pace, and head it down and across the keeper. Serve a mix of near-post, far-post, and cut-back height crosses. Rotate crosser, attacker, and keeper, and add a second attacker for near and far runs. Heading loads: no deliberate heading at U11 and under, and at U12-U13 keep it to roughly 15-20 headers in a session and under 30 minutes of heading a week, using a correctly sized and properly inflated ball (US Soccer and FA guidance).
Goalkeepers save flat headers comfortably and struggle badly with headers that bounce in front of them. A ball directed downwards hits the turf a yard or two before the line and comes up at an awkward height off an unpredictable surface, which is why heading down is coached everywhere and why it is worth explaining rather than just repeating.
Getting the forehead above the ball is what produces that trajectory, and it is a jumping and timing problem rather than a neck problem. A player who arrives underneath the ball can only head it up. A player who arrives with his head above its line at contact can head it into the ground.
Timing the run late is what makes that possible. Arriving early means standing under the flight, marked, and jumping from a stationary start, which produces almost no height. Arriving late and at pace means running onto the ball with momentum, and a running jump gets a player noticeably higher than a standing one.
Direction matters as much as trajectory. Heading across the keeper, towards the far side, means the ball travels away from where he is starting and he has the full width of the goal to cover. Heading back towards the near post plays it into the part of the goal he is already protecting.
The last habit is attacking the ball rather than waiting for it to drop into range, which is the same instruction as every other heading drill and is disobeyed most often here, because the ball is arriving from the side.
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.