You can train your players once a day. This is done by choosing "Training" in the top menu. On this page you can see which of your teams can train, or if they have already trained. Be mindful that players that have already trained in a different club will not be included in your daily training on that specific day.
When you click the "Train" button,you will be redirected to a results page where you can see how well your players performed in their training. The better the training is, the more the player will rise in attributes. If a player trains +150xp, that player will get 150xp in total, spread out in the attributes he is trained in.
By hovering over the players stats you will be able to see how many xp is required for the particular attribute. The players can have good and bad days, and there can be a big difference in a players training from day to day.
You also have the ability to see statistics about your players trainings, by going to Training > Statistics. Here you can choose which period you want to see. This can be useful to see a player's average training over a longer period.
The quality of your players' training depends of several factors. The most important are listed here:
Potential: A player's potential is crucial for training. The potential is something each players is "born with" and cannot be changed manually. However, the potential will alter slowly throughout a player's career.
Training facility: The better training facility, the better training.
Trainer: Hire a trainer to increase the quality of your training. The better trainer, the better training.
Intensity: The intensity you define under training groups has influence on training. The higher intensity, the better training and the higher loss of energy.
Energy: The higher a player's energy level is, the better training.
Mood: The higher mood, the better training.