Your players will develop as they play matches (or train in youth school). Those that are near their peak age and those with better potential will develop at faster pace.
Players in your main squad have to play at least one game in 3 days to get training bonus to their skills.
Each game they play also improves their experience.
Players in your youth squad can't play in professional matches but, in return, they get training bonus every day as if they played a match.
They will also improve their experience up to certain level as they prepare to join your main squad.
The optimal number of youth players is 10. Having smaller youth squad will improve their skills faster, but their overall training output will be lower.
Having more than 10 youth players will make them train slower and, again, reduce their overall training output.
Players will only improve their skills up to the maximum your training facilities allow.
Assistant manager improves that maximum for your main squad and youth coach for your youth squad.
The closer the player's skill to that maximum, the slower it improves.
It is, for example, faster to increase a skill from 1 to 2 than from 11 to 12.
Players that don't play a match in 3 days, players that are injured, or players that are past their peak could degrade in some of their skills.
The skill loss happens randomly and is more severe for older players.
Types of training
There are 8 types of training you can set for each of your players.
All training types can be used by all players, there is no wrong selection.
Only the skills relevant to the player's position will be influenced.
Improves the core skills related to player's general fitness and performance: reaction, strength, accuracy, ball control, set pieces, and fitness.
Affects skills responsible for maintaining possession and preventing opposition from doing the same: work rate, fitness, speed, ball control, technique, and accuracy.
Trains skills used for realising and preventing crosses indirect free kicks: tackling, positioning, work rate, passing, speed, set pieces.
Affects skills used for creating and preventing chances at the edge of the box: positioning, tackling, strength, passing, technique, vision.
One on ones.
Builds skills used for realising and preventing one on one chances: handling, marking, speed, technique, movement, and finishing.
Improves skills related to getting into position to shoot from long distances: handling, marking, positioning, accuracy, movement, and finishing.
Trains skills responsible for taking good positions to create or prevent headers and shots from within the box: handling, marking, strength, reaction, movement, and finishing.