There are only two ways of doing this, they have to spend money in training in the player academy and then super train these young players to the max spending a lot of greens to do so, then even when they lose 20% of the end of the season they still have them very high skills. Remember that you can put a young player in the youth academy that is always very high in skills if they spent a lot of tokens and resources in the youth academy then you take that player and stick him in the player training academy and buy the ONE DAY training and voila, you have an 18-year-old with very high skills for his role. That's how they do it and they have to spend a lot of money in order to do so because you cannot do so without spending a lot of resources and or money, both really.