Here are a couple of different heading exercises for soccer to teach heading from crosses.
A coaching point would be that players should make bending runs to goal so they can open their hips and face the ball coming in from the wing. Most players will start out by running straight to goal, and you have to train away from this, starting as soon as you have realistic ball services from the side.
A second coaching point for the 11 steps preceding is that these services require the player to attack the ball, and most of the services would be lofted balls, rather than driven balls to the near post.
Another tip that may help you if you move on to near post headers with driven balls (as opposed to lofted balls at the back post that the 11 steps above cover): If you do not have the consistent skill to serve a driven ball to the near post and get the timing just right and not strike it so hard that you kill one of your starting players, there is a workaround. Lie on the ground (face up) near the edge of the goal box near the goal-line (on the side of the goal-area). In this position, you can throw balls with an overhead throw from this position that have about the right trajectory and pace, and which always have the correct location. Not perfect, but if you hit the same spot each time, your players will start redirecting balls into the goal with confidence.