If you buy a player young and his training level is bad. How can you get the training up

Gold balls...

you cant increase his training speed

How do you use gold balls on the player

You don't use it on the player, you use it for quick training

you can improve the training efficiency by upgrading your training center, but you can't increase individual players training speed, tha why you must always look for players with the best training speed. i personally now only buy new players with a 150% minimum training speed, then i look for a minimum of 3.00 recovery rate and a max age of 20y, i don't worry to much about their stars as long they have super high recovery and training rates