I agree with a lot of training methods that people have suggested above, for my part i would just like to add one important thing that i would try and tell the players that i’m gonna be working with. Ideally the players should just try and focus on hitting the target trying to forget about the goal keeper. I would then ask them to visualise possible negative outcomes of the penalty without the goal keeper. The focus then would be threefold
Hitting the desired part of the ball, so focusing on where they want to make contact with the ball.
Making sure that they don’t slip on the final step right before contact with the ball. Being able to play out the whole scenario of making contact with the ball till it hits the back of the net. This would be done before training for a penalty as well as after the penalty in order to see if they succeed in picturing it all in their heads as well as they executed it