Training a youth player isn't too complicated. More depend from lucky than our's 'skills'. Most important thing is sending to Training Camp every season. In my opinion youth packets are useless.
What skills we should training depend of youths looks at start, but I always rather train a main skill(tackling,shooting) to about 8balls - ofcourse sending this players to TC every season so he train speed, stamina and ball control meantime. If player maxed earlier sense of invest for him depend of how much balls he have...
Different of this are midfielders which must has more skills so I avoid train one skill to 8balls, and I try train more skills to 5-6balls then check max in this skill and train next.
Keepers are very simply but very risk instance so I rather avoid train keepers.
Most important skills for anyone youths, which must be training, are(priority sequence):
defenders:
tackling,speed,stamina,ball control
midfielders:
speed, stamina, ball control, passing, tackling and play inteligence
strikers:
shooting,speed, ball control and stamina.