In Two Point Hospital, you can train your staff. In order to do that, you will need a special room called "training". One training room can be used by only one trainer at a given moment. Thus, you should build at least two training rooms.
Your employees need training to get promoted, so you should train them regularly until they reach the maximal level. Otherwise, their satisfaction (due to low salary) will increase significantly. The number of available training sessions depends on the experience of the hired employee.
It's best to train your employees after they get promoted and a new skill slot is unlocked. This way you will fully use your staff's abilities. You can increase your employee's effectiveness, and select a specific moment when you want to send him/her on a training. If you wait too long with the training, you will be notified that further education is required and you will be forced with the procedure if you want to promote your employee.