Group demonstration, then scout all the 5* skill players. You can scout the 4* skill players if needed, but only if you have extra scouting points. All the 3* and below aren't worth it.
I can't exactly remember the details, but don't go for players listed as SFs. They are most likely to have terrible starting skills
Right now 19 year olds have a boost in skills, so either 18 year olds or 19 year olds would be fine, as long as it's 5*. If you aren't aiming for 5* potential and just want to train for the U21, then go for 19 year old 5*.
That's all I can remember for now.