I would also like to add the following.
I advise you not to train many players to gain a star, especially if they are in the lower 4 star region. This will be too expensive (unless you have a lot of money to spend, or you can get enough tokens from offers). You can train the best performers (both in terms of training and performance on the pitch), but don't go beyond 3 or 4 players. If you want better players, sell some of your current players and buy better quality players from the market.