main idea is like that..
1.put them in u18 until 18 age
2. get a good tutor
3. age 17-> consider a loan
4.age 18-> consider loan teams to higher level competition(epl, la liga etc)
5.age 19-> make them a sub in your team, play them in some matches, and sub them for 20 minutes.
age 20 should able to be a good sub for him position.