To make a superstar you have to complete a tier with enough players to fill the appropriate superstar bar for that tier.
Example,to make a s*QB/HB/LB or 150 tier superstar you have to have 20 completed seasons total from that group. So in total 20 QB/HB/LB at end of season become sophomore you Will earn a 150 tier superstar to make next season. Or if you have only 5 total, soon as they complete 4 seasons you will earn it.
15 for the 100 tier and 10 for the 50 tier.