Each star is about 5Qulity points which are 45 skill points you can earn from training , matches , and friendlies

you can see your player average quality as I'm level 2 my 4star player is from avg 20 to 24
if he got to 25 avg. he will be a 5 star player and so on ..
he starts 25 at star 5 if he reaches 30 he will be a 6 star player " Scout player "